For a FUSE_COMPOUND we add a header that contains information
about how many commands there are in the compound and about the
size of the expected result. This will make the interpretation
in libfuse easier, since we can preallocate the whole result.
Then we append the requests that belong to this compound.
The API for the compound command has:
fuse_compound_alloc()
fuse_compound_add()
fuse_compound_request()
fuse_compound_free()
Signed-off-by: Horst Birthelmer <hbirthelmer@ddn.com>
Tested-by: syzbot@syzkaller.appspotmail.com
---
fs/fuse/Makefile | 2 +-
fs/fuse/compound.c | 368 ++++++++++++++++++++++++++++++++++++++++++++++
fs/fuse/dev.c | 25 ++++
fs/fuse/fuse_i.h | 14 ++
include/uapi/linux/fuse.h | 37 +++++
5 files changed, 445 insertions(+), 1 deletion(-)
diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
index 22ad9538dfc4..4c09038ef995 100644
--- a/fs/fuse/Makefile
+++ b/fs/fuse/Makefile
@@ -11,7 +11,7 @@ obj-$(CONFIG_CUSE) += cuse.o
obj-$(CONFIG_VIRTIO_FS) += virtiofs.o
fuse-y := trace.o # put trace.o first so we see ftrace errors sooner
-fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o
+fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o compound.o
fuse-y += iomode.o
fuse-$(CONFIG_FUSE_DAX) += dax.o
fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o backing.o
diff --git a/fs/fuse/compound.c b/fs/fuse/compound.c
new file mode 100644
index 000000000000..b15014d61b38
--- /dev/null
+++ b/fs/fuse/compound.c
@@ -0,0 +1,368 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * FUSE: Filesystem in Userspace
+ * Copyright (C) 2025
+ *
+ * This file implements compound operations for FUSE, allowing multiple
+ * operations to be batched into a single request to reduce round trips
+ * between kernel and userspace.
+ */
+
+#include "fuse_i.h"
+
+/*
+ * Compound request builder and state tracker
+ *
+ * This structure manages the lifecycle of a compound FUSE request, from building
+ * the request by serializing multiple operations into a single buffer, through
+ * sending it to userspace, to parsing the compound response back into individual
+ * operation results.
+ */
+struct fuse_compound_req {
+ struct fuse_mount *fm;
+ struct fuse_compound_in compound_header;
+ struct fuse_compound_out result_header;
+
+ size_t total_size; /* Total size of serialized operations */
+ char *buffer; /* Buffer holding serialized requests */
+ size_t buffer_pos; /* Current write position in buffer */
+ size_t buffer_size; /* Total allocated buffer size */
+
+ size_t total_expected_out_size; /* Sum of expected output sizes */
+
+ /* Per-operation error codes */
+ int op_errors[FUSE_MAX_COMPOUND_OPS];
+ /* Original fuse_args for response parsing */
+ struct fuse_args *op_args[FUSE_MAX_COMPOUND_OPS];
+
+ bool parsed; /* Prevent double-parsing of response */
+};
+
+struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm,
+ uint32_t flags)
+{
+ struct fuse_compound_req *compound;
+
+ compound = kzalloc(sizeof(*compound), GFP_KERNEL);
+ if (!compound)
+ return ERR_PTR(-ENOMEM);
+
+ compound->fm = fm;
+ compound->compound_header.flags = flags;
+ compound->buffer_size = PAGE_SIZE;
+ compound->buffer = kvmalloc(compound->buffer_size, GFP_KERNEL);
+ if (!compound->buffer) {
+ kfree(compound);
+ return ERR_PTR(-ENOMEM);
+ }
+ return compound;
+}
+
+void fuse_compound_free(struct fuse_compound_req *compound)
+{
+ if (!compound)
+ return;
+
+ kvfree(compound->buffer);
+ kfree(compound);
+}
+
+static int fuse_compound_validate_header(struct fuse_compound_req *compound)
+{
+ struct fuse_compound_in *in_header = &compound->compound_header;
+ size_t offset = 0;
+ int i;
+
+ if (compound->buffer_pos > compound->buffer_size)
+ return -EINVAL;
+
+ if (!compound || !compound->buffer)
+ return -EINVAL;
+
+ if (compound->buffer_pos < sizeof(struct fuse_in_header))
+ return -EINVAL;
+
+ if (in_header->count == 0 || in_header->count > FUSE_MAX_COMPOUND_OPS)
+ return -EINVAL;
+
+ for (i = 0; i < in_header->count; i++) {
+ const struct fuse_in_header *op_hdr;
+
+ if (offset + sizeof(struct fuse_in_header) >
+ compound->buffer_pos) {
+ pr_info_ratelimited("FUSE: compound operation %d header extends beyond buffer (offset %zu + header size %zu > buffer pos %zu)\n",
+ i, offset,
+ sizeof(struct fuse_in_header),
+ compound->buffer_pos);
+ return -EINVAL;
+ }
+
+ op_hdr = (const struct fuse_in_header *)(compound->buffer +
+ offset);
+
+ if (op_hdr->len < sizeof(struct fuse_in_header)) {
+ pr_info_ratelimited("FUSE: compound operation %d has invalid length %u (minimum %zu bytes)\n",
+ i, op_hdr->len,
+ sizeof(struct fuse_in_header));
+ return -EINVAL;
+ }
+
+ if (offset + op_hdr->len > compound->buffer_pos) {
+ pr_info_ratelimited("FUSE: compound operation %d extends beyond buffer (offset %zu + length %u > buffer pos %zu)\n",
+ i, offset, op_hdr->len,
+ compound->buffer_pos);
+ return -EINVAL;
+ }
+
+ if (op_hdr->opcode == 0 || op_hdr->opcode == FUSE_COMPOUND) {
+ pr_info_ratelimited("FUSE: compound operation %d has invalid opcode %u (cannot be 0 or FUSE_COMPOUND)\n",
+ i, op_hdr->opcode);
+ return -EINVAL;
+ }
+
+ if (op_hdr->nodeid == 0) {
+ pr_info_ratelimited("FUSE: compound operation %d has invalid node ID 0\n",
+ i);
+ return -EINVAL;
+ }
+
+ offset += op_hdr->len;
+ }
+
+ if (offset != compound->buffer_pos) {
+ pr_info_ratelimited("FUSE: compound buffer size mismatch (calculated %zu bytes, actual %zu bytes)\n",
+ offset, compound->buffer_pos);
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
+int fuse_compound_add(struct fuse_compound_req *compound,
+ struct fuse_args *args)
+{
+ struct fuse_in_header *hdr;
+ size_t args_size = 0;
+ size_t needed_size;
+ size_t expected_out_size = 0;
+ int i;
+
+ if (!compound ||
+ compound->compound_header.count >= FUSE_MAX_COMPOUND_OPS)
+ return -EINVAL;
+
+ if (args->in_pages)
+ return -EINVAL;
+
+ for (i = 0; i < args->in_numargs; i++)
+ args_size += args->in_args[i].size;
+
+ for (i = 0; i < args->out_numargs; i++)
+ expected_out_size += args->out_args[i].size;
+
+ needed_size = sizeof(struct fuse_in_header) + args_size;
+
+ if (compound->buffer_pos + needed_size > compound->buffer_size) {
+ size_t new_size = max(compound->buffer_size * 2,
+ compound->buffer_pos + needed_size);
+ char *new_buffer;
+
+ new_size = round_up(new_size, PAGE_SIZE);
+ new_buffer = kvrealloc(compound->buffer, new_size,
+ GFP_KERNEL);
+ if (!new_buffer)
+ return -ENOMEM;
+ compound->buffer = new_buffer;
+ compound->buffer_size = new_size;
+ }
+
+ /* Build request header */
+ hdr = (struct fuse_in_header *)(compound->buffer +
+ compound->buffer_pos);
+ memset(hdr, 0, sizeof(*hdr));
+ hdr->len = needed_size;
+ hdr->opcode = args->opcode;
+ hdr->nodeid = args->nodeid;
+ hdr->uid = from_kuid(compound->fm->fc->user_ns, current_fsuid());
+ hdr->gid = from_kgid(compound->fm->fc->user_ns, current_fsgid());
+ hdr->pid = pid_nr_ns(task_pid(current), compound->fm->fc->pid_ns);
+ hdr->unique = fuse_get_unique(&compound->fm->fc->iq);
+ compound->buffer_pos += sizeof(*hdr);
+
+ for (i = 0; i < args->in_numargs; i++) {
+ memcpy(compound->buffer + compound->buffer_pos,
+ args->in_args[i].value, args->in_args[i].size);
+ compound->buffer_pos += args->in_args[i].size;
+ }
+
+ compound->total_expected_out_size += expected_out_size;
+
+ /* Store args for response parsing */
+ compound->op_args[compound->compound_header.count] = args;
+
+ compound->compound_header.count++;
+ compound->total_size += needed_size;
+
+ return 0;
+}
+
+static void *fuse_copy_response_data(struct fuse_args *args,
+ char *response_data)
+{
+ size_t copied = 0;
+ int arg_idx;
+
+ for (arg_idx = 0; arg_idx < args->out_numargs; arg_idx++) {
+ struct fuse_arg current_arg = args->out_args[arg_idx];
+ size_t arg_size;
+
+ /* Last argument with out_pages: copy to pages */
+ if (arg_idx == args->out_numargs - 1 && args->out_pages) {
+ /*
+ * External payload (in the last out arg)
+ * is not supported at the moment
+ */
+ return response_data;
+ }
+
+ arg_size = current_arg.size;
+
+ if (current_arg.value && arg_size > 0) {
+ memcpy(current_arg.value,
+ (char *)response_data + copied,
+ arg_size);
+ copied += arg_size;
+ }
+ }
+
+ return (char *)response_data + copied;
+}
+
+int fuse_compound_get_error(struct fuse_compound_req *compound, int op_idx)
+{
+ return compound->op_errors[op_idx];
+}
+
+static void *fuse_compound_parse_one_op(struct fuse_compound_req *compound,
+ int op_index, void *op_out_data,
+ void *response_end)
+{
+ struct fuse_out_header *op_hdr = op_out_data;
+ struct fuse_args *args = compound->op_args[op_index];
+
+ if (op_hdr->len < sizeof(struct fuse_out_header))
+ return NULL;
+
+ /* Check if the entire operation response fits in the buffer */
+ if ((char *)op_out_data + op_hdr->len > (char *)response_end)
+ return NULL;
+
+ if (op_hdr->error != 0)
+ compound->op_errors[op_index] = op_hdr->error;
+
+ if (args && op_hdr->len > sizeof(struct fuse_out_header))
+ return fuse_copy_response_data(args, op_out_data +
+ sizeof(struct fuse_out_header));
+
+ /* No response data, just advance past the header */
+ return (char *)op_out_data + op_hdr->len;
+}
+
+static int fuse_compound_parse_resp(struct fuse_compound_req *compound,
+ uint32_t count, void *response,
+ size_t response_size)
+{
+ void *op_out_data = response;
+ void *response_end = (char *)response + response_size;
+ int i;
+
+ if (compound->parsed)
+ return 0;
+
+ if (!response || response_size < sizeof(struct fuse_out_header))
+ return -EIO;
+
+ for (i = 0; i < count && i < compound->result_header.count; i++) {
+ op_out_data = fuse_compound_parse_one_op(compound, i,
+ op_out_data,
+ response_end);
+ if (!op_out_data)
+ return -EIO;
+ }
+
+ compound->parsed = true;
+ return 0;
+}
+
+ssize_t fuse_compound_send(struct fuse_compound_req *compound)
+{
+ struct fuse_args args = {
+ .opcode = FUSE_COMPOUND,
+ .nodeid = 0,
+ .in_numargs = 2,
+ .out_numargs = 2,
+ .out_argvar = true,
+ };
+ size_t expected_response_size;
+ size_t total_buffer_size;
+ size_t actual_response_size;
+ void *resp_payload;
+ ssize_t ret;
+
+ if (!compound) {
+ pr_info_ratelimited("FUSE: compound request is NULL in %s\n",
+ __func__);
+ return -EINVAL;
+ }
+
+ if (compound->compound_header.count == 0) {
+ pr_info_ratelimited("FUSE: compound request contains no operations\n");
+ return -EINVAL;
+ }
+
+ expected_response_size = compound->total_expected_out_size;
+ total_buffer_size = expected_response_size +
+ (compound->compound_header.count *
+ sizeof(struct fuse_out_header));
+
+ resp_payload = kvmalloc(total_buffer_size, GFP_KERNEL | __GFP_ZERO);
+ if (!resp_payload)
+ return -ENOMEM;
+
+ compound->compound_header.result_size = expected_response_size;
+
+ args.in_args[0].size = sizeof(compound->compound_header);
+ args.in_args[0].value = &compound->compound_header;
+ args.in_args[1].size = compound->buffer_pos;
+ args.in_args[1].value = compound->buffer;
+
+ args.out_args[0].size = sizeof(compound->result_header);
+ args.out_args[0].value = &compound->result_header;
+ args.out_args[1].size = total_buffer_size;
+ args.out_args[1].value = resp_payload;
+
+ ret = fuse_compound_validate_header(compound);
+ if (ret)
+ goto out;
+
+ ret = fuse_compound_request(compound->fm, &args);
+ if (ret < 0)
+ goto out;
+
+ actual_response_size = args.out_args[1].size;
+
+ if (actual_response_size < sizeof(struct fuse_compound_out)) {
+ pr_info_ratelimited("FUSE: compound response too small (%zu bytes, minimum %zu bytes)\n",
+ actual_response_size,
+ sizeof(struct fuse_compound_out));
+ ret = -EINVAL;
+ goto out;
+ }
+
+ ret = fuse_compound_parse_resp(compound, compound->result_header.count,
+ (char *)resp_payload,
+ actual_response_size);
+out:
+ kvfree(resp_payload);
+ return ret;
+}
diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 6d59cbc877c6..2d89ca69308f 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -660,6 +660,31 @@ static void fuse_args_to_req(struct fuse_req *req, struct fuse_args *args)
__set_bit(FR_ASYNC, &req->flags);
}
+ssize_t fuse_compound_request(
+ struct fuse_mount *fm, struct fuse_args *args)
+{
+ struct fuse_req *req;
+ ssize_t ret;
+
+ req = fuse_get_req(&invalid_mnt_idmap, fm, false);
+ if (IS_ERR(req))
+ return PTR_ERR(req);
+
+ fuse_args_to_req(req, args);
+
+ if (!args->noreply)
+ __set_bit(FR_ISREPLY, &req->flags);
+
+ __fuse_request_send(req);
+ ret = req->out.h.error;
+ if (!ret && args->out_argvar) {
+ BUG_ON(args->out_numargs == 0);
+ ret = args->out_args[args->out_numargs - 1].size;
+ }
+ fuse_put_request(req);
+ return ret;
+}
+
ssize_t __fuse_simple_request(struct mnt_idmap *idmap,
struct fuse_mount *fm,
struct fuse_args *args)
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 7f16049387d1..86253517f59b 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1273,6 +1273,20 @@ static inline ssize_t fuse_simple_idmap_request(struct mnt_idmap *idmap,
int fuse_simple_background(struct fuse_mount *fm, struct fuse_args *args,
gfp_t gfp_flags);
+/**
+ * Compound request API
+ */
+struct fuse_compound_req;
+ssize_t fuse_compound_request(struct fuse_mount *fm, struct fuse_args *args);
+ssize_t fuse_compound_send(struct fuse_compound_req *compound);
+
+struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, uint32_t flags);
+int fuse_compound_add(struct fuse_compound_req *compound,
+ struct fuse_args *args);
+int fuse_compound_get_error(struct fuse_compound_req *compound,
+ int op_idx);
+void fuse_compound_free(struct fuse_compound_req *compound);
+
/**
* Assign a unique id to a fuse request
*/
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index c13e1f9a2f12..848323acecdc 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -664,6 +664,13 @@ enum fuse_opcode {
FUSE_STATX = 52,
FUSE_COPY_FILE_RANGE_64 = 53,
+ /* A compound request works like multiple simple requests.
+ * This is a special case for calls that can be combined atomic on the
+ * fuse server. If the server actually does atomically execute the command is
+ * left to the fuse server implementation.
+ */
+ FUSE_COMPOUND = 101,
+
/* CUSE specific operations */
CUSE_INIT = 4096,
@@ -1245,6 +1252,36 @@ struct fuse_supp_groups {
uint32_t groups[];
};
+#define FUSE_MAX_COMPOUND_OPS 16 /* Maximum operations per compound */
+
+/*
+ * Compound request header
+ *
+ * This header is followed by the fuse requests
+ */
+struct fuse_compound_in {
+ uint32_t count; /* Number of operations */
+ uint32_t flags; /* Compound flags */
+
+ /* Total size of all results.
+ * This is needed for preallocating the whole result for all
+ * commands in this compound.
+ */
+ uint32_t result_size;
+ uint64_t reserved;
+};
+
+/*
+ * Compound response header
+ *
+ * This header is followed by complete fuse responses
+ */
+struct fuse_compound_out {
+ uint32_t count; /* Number of results */
+ uint32_t flags; /* Result flags */
+ uint64_t reserved;
+};
+
/**
* Size of the ring buffer header
*/
--
2.51.0On Tue, Dec 23, 2025 at 2:13 PM Horst Birthelmer
<hbirthelmer@googlemail.com> wrote:
>
> For a FUSE_COMPOUND we add a header that contains information
> about how many commands there are in the compound and about the
> size of the expected result. This will make the interpretation
> in libfuse easier, since we can preallocate the whole result.
> Then we append the requests that belong to this compound.
>
> The API for the compound command has:
> fuse_compound_alloc()
> fuse_compound_add()
> fuse_compound_request()
> fuse_compound_free()
>
> Signed-off-by: Horst Birthelmer <hbirthelmer@ddn.com>
> Tested-by: syzbot@syzkaller.appspotmail.com
> ---
> fs/fuse/Makefile | 2 +-
> fs/fuse/compound.c | 368 ++++++++++++++++++++++++++++++++++++++++++++++
> fs/fuse/dev.c | 25 ++++
> fs/fuse/fuse_i.h | 14 ++
> include/uapi/linux/fuse.h | 37 +++++
> 5 files changed, 445 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
> index 22ad9538dfc4..4c09038ef995 100644
> --- a/fs/fuse/Makefile
> +++ b/fs/fuse/Makefile
> @@ -11,7 +11,7 @@ obj-$(CONFIG_CUSE) += cuse.o
> obj-$(CONFIG_VIRTIO_FS) += virtiofs.o
>
> fuse-y := trace.o # put trace.o first so we see ftrace errors sooner
> -fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o
> +fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o compound.o
> fuse-y += iomode.o
> fuse-$(CONFIG_FUSE_DAX) += dax.o
> fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o backing.o
> diff --git a/fs/fuse/compound.c b/fs/fuse/compound.c
> new file mode 100644
> index 000000000000..b15014d61b38
> --- /dev/null
> +++ b/fs/fuse/compound.c
> @@ -0,0 +1,368 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * FUSE: Filesystem in Userspace
> + * Copyright (C) 2025
> + *
> + * This file implements compound operations for FUSE, allowing multiple
> + * operations to be batched into a single request to reduce round trips
> + * between kernel and userspace.
> + */
> +
> +#include "fuse_i.h"
> +
> +/*
> + * Compound request builder and state tracker
> + *
> + * This structure manages the lifecycle of a compound FUSE request, from building
> + * the request by serializing multiple operations into a single buffer, through
> + * sending it to userspace, to parsing the compound response back into individual
> + * operation results.
> + */
> +struct fuse_compound_req {
> + struct fuse_mount *fm;
> + struct fuse_compound_in compound_header;
> + struct fuse_compound_out result_header;
> +
> + size_t total_size; /* Total size of serialized operations */
> + char *buffer; /* Buffer holding serialized requests */
> + size_t buffer_pos; /* Current write position in buffer */
> + size_t buffer_size; /* Total allocated buffer size */
> +
> + size_t total_expected_out_size; /* Sum of expected output sizes */
> +
> + /* Per-operation error codes */
> + int op_errors[FUSE_MAX_COMPOUND_OPS];
> + /* Original fuse_args for response parsing */
> + struct fuse_args *op_args[FUSE_MAX_COMPOUND_OPS];
> +
> + bool parsed; /* Prevent double-parsing of response */
> +};
> +
> +struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm,
> + uint32_t flags)
> +{
> + struct fuse_compound_req *compound;
> +
> + compound = kzalloc(sizeof(*compound), GFP_KERNEL);
> + if (!compound)
> + return ERR_PTR(-ENOMEM);
> +
> + compound->fm = fm;
> + compound->compound_header.flags = flags;
> + compound->buffer_size = PAGE_SIZE;
> + compound->buffer = kvmalloc(compound->buffer_size, GFP_KERNEL);
> + if (!compound->buffer) {
> + kfree(compound);
> + return ERR_PTR(-ENOMEM);
> + }
> + return compound;
> +}
> +
> +void fuse_compound_free(struct fuse_compound_req *compound)
> +{
> + if (!compound)
> + return;
> +
> + kvfree(compound->buffer);
> + kfree(compound);
> +}
> +
> +static int fuse_compound_validate_header(struct fuse_compound_req *compound)
> +{
> + struct fuse_compound_in *in_header = &compound->compound_header;
> + size_t offset = 0;
> + int i;
> +
> + if (compound->buffer_pos > compound->buffer_size)
> + return -EINVAL;
> +
> + if (!compound || !compound->buffer)
> + return -EINVAL;
> +
> + if (compound->buffer_pos < sizeof(struct fuse_in_header))
> + return -EINVAL;
> +
> + if (in_header->count == 0 || in_header->count > FUSE_MAX_COMPOUND_OPS)
> + return -EINVAL;
> +
> + for (i = 0; i < in_header->count; i++) {
> + const struct fuse_in_header *op_hdr;
> +
> + if (offset + sizeof(struct fuse_in_header) >
> + compound->buffer_pos) {
> + pr_info_ratelimited("FUSE: compound operation %d header extends beyond buffer (offset %zu + header size %zu > buffer pos %zu)\n",
> + i, offset,
> + sizeof(struct fuse_in_header),
> + compound->buffer_pos);
> + return -EINVAL;
> + }
> +
> + op_hdr = (const struct fuse_in_header *)(compound->buffer +
> + offset);
> +
> + if (op_hdr->len < sizeof(struct fuse_in_header)) {
> + pr_info_ratelimited("FUSE: compound operation %d has invalid length %u (minimum %zu bytes)\n",
> + i, op_hdr->len,
> + sizeof(struct fuse_in_header));
> + return -EINVAL;
> + }
> +
> + if (offset + op_hdr->len > compound->buffer_pos) {
> + pr_info_ratelimited("FUSE: compound operation %d extends beyond buffer (offset %zu + length %u > buffer pos %zu)\n",
> + i, offset, op_hdr->len,
> + compound->buffer_pos);
> + return -EINVAL;
> + }
> +
> + if (op_hdr->opcode == 0 || op_hdr->opcode == FUSE_COMPOUND) {
> + pr_info_ratelimited("FUSE: compound operation %d has invalid opcode %u (cannot be 0 or FUSE_COMPOUND)\n",
> + i, op_hdr->opcode);
> + return -EINVAL;
> + }
> +
> + if (op_hdr->nodeid == 0) {
> + pr_info_ratelimited("FUSE: compound operation %d has invalid node ID 0\n",
> + i);
> + return -EINVAL;
> + }
> +
> + offset += op_hdr->len;
> + }
> +
> + if (offset != compound->buffer_pos) {
> + pr_info_ratelimited("FUSE: compound buffer size mismatch (calculated %zu bytes, actual %zu bytes)\n",
> + offset, compound->buffer_pos);
> + return -EINVAL;
> + }
> +
> + return 0;
I wonder if this is overkill to have given that the kernel is in
charge of setting up the compound request and if the code is right,
has done it correctly. imo it adds extra overhead but doesn't offer
much given that the kernel code should aalready be correct.
> +}
> +
> +int fuse_compound_add(struct fuse_compound_req *compound,
> + struct fuse_args *args)
> +{
> + struct fuse_in_header *hdr;
> + size_t args_size = 0;
> + size_t needed_size;
> + size_t expected_out_size = 0;
> + int i;
> +
> + if (!compound ||
> + compound->compound_header.count >= FUSE_MAX_COMPOUND_OPS)
> + return -EINVAL;
> +
> + if (args->in_pages)
> + return -EINVAL;
> +
> + for (i = 0; i < args->in_numargs; i++)
> + args_size += args->in_args[i].size;
> +
> + for (i = 0; i < args->out_numargs; i++)
> + expected_out_size += args->out_args[i].size;
> +
> + needed_size = sizeof(struct fuse_in_header) + args_size;
> +
> + if (compound->buffer_pos + needed_size > compound->buffer_size) {
> + size_t new_size = max(compound->buffer_size * 2,
> + compound->buffer_pos + needed_size);
> + char *new_buffer;
> +
> + new_size = round_up(new_size, PAGE_SIZE);
> + new_buffer = kvrealloc(compound->buffer, new_size,
> + GFP_KERNEL);
> + if (!new_buffer)
> + return -ENOMEM;
> + compound->buffer = new_buffer;
> + compound->buffer_size = new_size;
Hmm... when we're setting up a compound request, we already know the
size that will be needed to hold all the requests, right? Do you think
it makes sense to allocate that from the get-go in
fuse_compound_alloc() and then not have to do any buffer reallocation?
I think that also gets rid of fuse_compound_req->total_size, as that
would just be the same as fuse_compound_req->buffer_size.
> + }
> +
> + /* Build request header */
> + hdr = (struct fuse_in_header *)(compound->buffer +
> + compound->buffer_pos);
> + memset(hdr, 0, sizeof(*hdr));
> + hdr->len = needed_size;
> + hdr->opcode = args->opcode;
> + hdr->nodeid = args->nodeid;
> + hdr->uid = from_kuid(compound->fm->fc->user_ns, current_fsuid());
> + hdr->gid = from_kgid(compound->fm->fc->user_ns, current_fsgid());
> + hdr->pid = pid_nr_ns(task_pid(current), compound->fm->fc->pid_ns);
> + hdr->unique = fuse_get_unique(&compound->fm->fc->iq);
> + compound->buffer_pos += sizeof(*hdr);
> +
> + for (i = 0; i < args->in_numargs; i++) {
> + memcpy(compound->buffer + compound->buffer_pos,
> + args->in_args[i].value, args->in_args[i].size);
> + compound->buffer_pos += args->in_args[i].size;
> + }
> +
> + compound->total_expected_out_size += expected_out_size;
> +
> + /* Store args for response parsing */
> + compound->op_args[compound->compound_header.count] = args;
> +
> + compound->compound_header.count++;
> + compound->total_size += needed_size;
> +
> + return 0;
Does fc->max_pages need to be accounted for as well? iirc, that's the
upper limit on how many pages can be forwarded to the server in a
request.
> +}
> +
> +static void *fuse_copy_response_data(struct fuse_args *args,
> + char *response_data)
> +{
> + size_t copied = 0;
> + int arg_idx;
> +
> + for (arg_idx = 0; arg_idx < args->out_numargs; arg_idx++) {
> + struct fuse_arg current_arg = args->out_args[arg_idx];
> + size_t arg_size;
> +
> + /* Last argument with out_pages: copy to pages */
> + if (arg_idx == args->out_numargs - 1 && args->out_pages) {
> + /*
> + * External payload (in the last out arg)
> + * is not supported at the moment
> + */
> + return response_data;
> + }
> +
> + arg_size = current_arg.size;
> +
> + if (current_arg.value && arg_size > 0) {
> + memcpy(current_arg.value,
> + (char *)response_data + copied,
> + arg_size);
> + copied += arg_size;
> + }
> + }
> +
> + return (char *)response_data + copied;
> +}
> +
> +int fuse_compound_get_error(struct fuse_compound_req *compound, int op_idx)
> +{
> + return compound->op_errors[op_idx];
> +}
> +
> +static void *fuse_compound_parse_one_op(struct fuse_compound_req *compound,
> + int op_index, void *op_out_data,
> + void *response_end)
> +{
> + struct fuse_out_header *op_hdr = op_out_data;
> + struct fuse_args *args = compound->op_args[op_index];
> +
> + if (op_hdr->len < sizeof(struct fuse_out_header))
> + return NULL;
> +
> + /* Check if the entire operation response fits in the buffer */
> + if ((char *)op_out_data + op_hdr->len > (char *)response_end)
> + return NULL;
> +
> + if (op_hdr->error != 0)
> + compound->op_errors[op_index] = op_hdr->error;
> +
> + if (args && op_hdr->len > sizeof(struct fuse_out_header))
> + return fuse_copy_response_data(args, op_out_data +
> + sizeof(struct fuse_out_header));
> +
> + /* No response data, just advance past the header */
> + return (char *)op_out_data + op_hdr->len;
> +}
> +
> +static int fuse_compound_parse_resp(struct fuse_compound_req *compound,
> + uint32_t count, void *response,
> + size_t response_size)
> +{
> + void *op_out_data = response;
> + void *response_end = (char *)response + response_size;
> + int i;
> +
> + if (compound->parsed)
> + return 0;
> +
> + if (!response || response_size < sizeof(struct fuse_out_header))
> + return -EIO;
> +
> + for (i = 0; i < count && i < compound->result_header.count; i++) {
> + op_out_data = fuse_compound_parse_one_op(compound, i,
> + op_out_data,
> + response_end);
> + if (!op_out_data)
> + return -EIO;
> + }
> +
> + compound->parsed = true;
> + return 0;
> +}
> +
> +ssize_t fuse_compound_send(struct fuse_compound_req *compound)
> +{
> + struct fuse_args args = {
> + .opcode = FUSE_COMPOUND,
> + .nodeid = 0,
> + .in_numargs = 2,
> + .out_numargs = 2,
> + .out_argvar = true,
> + };
> + size_t expected_response_size;
> + size_t total_buffer_size;
> + size_t actual_response_size;
> + void *resp_payload;
> + ssize_t ret;
> +
> + if (!compound) {
> + pr_info_ratelimited("FUSE: compound request is NULL in %s\n",
> + __func__);
> + return -EINVAL;
> + }
> +
> + if (compound->compound_header.count == 0) {
> + pr_info_ratelimited("FUSE: compound request contains no operations\n");
> + return -EINVAL;
> + }
> +
> + expected_response_size = compound->total_expected_out_size;
> + total_buffer_size = expected_response_size +
> + (compound->compound_header.count *
> + sizeof(struct fuse_out_header));
> +
> + resp_payload = kvmalloc(total_buffer_size, GFP_KERNEL | __GFP_ZERO);
> + if (!resp_payload)
> + return -ENOMEM;
> +
> + compound->compound_header.result_size = expected_response_size;
> +
> + args.in_args[0].size = sizeof(compound->compound_header);
> + args.in_args[0].value = &compound->compound_header;
> + args.in_args[1].size = compound->buffer_pos;
> + args.in_args[1].value = compound->buffer;
> +
> + args.out_args[0].size = sizeof(compound->result_header);
> + args.out_args[0].value = &compound->result_header;
> + args.out_args[1].size = total_buffer_size;
> + args.out_args[1].value = resp_payload;
> +
> + ret = fuse_compound_validate_header(compound);
> + if (ret)
> + goto out;
> +
> + ret = fuse_compound_request(compound->fm, &args);
> + if (ret < 0)
> + goto out;
> +
> + actual_response_size = args.out_args[1].size;
> +
> + if (actual_response_size < sizeof(struct fuse_compound_out)) {
> + pr_info_ratelimited("FUSE: compound response too small (%zu bytes, minimum %zu bytes)\n",
> + actual_response_size,
> + sizeof(struct fuse_compound_out));
> + ret = -EINVAL;
> + goto out;
> + }
> +
> + ret = fuse_compound_parse_resp(compound, compound->result_header.count,
> + (char *)resp_payload,
> + actual_response_size);
> +out:
> + kvfree(resp_payload);
> + return ret;
> +}
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 6d59cbc877c6..2d89ca69308f 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -660,6 +660,31 @@ static void fuse_args_to_req(struct fuse_req *req, struct fuse_args *args)
> __set_bit(FR_ASYNC, &req->flags);
> }
>
> +ssize_t fuse_compound_request(
> + struct fuse_mount *fm, struct fuse_args *args)
> +{
> + struct fuse_req *req;
> + ssize_t ret;
> +
> + req = fuse_get_req(&invalid_mnt_idmap, fm, false);
> + if (IS_ERR(req))
> + return PTR_ERR(req);
> +
> + fuse_args_to_req(req, args);
> +
> + if (!args->noreply)
> + __set_bit(FR_ISREPLY, &req->flags);
> +
> + __fuse_request_send(req);
> + ret = req->out.h.error;
> + if (!ret && args->out_argvar) {
> + BUG_ON(args->out_numargs == 0);
> + ret = args->out_args[args->out_numargs - 1].size;
> + }
> + fuse_put_request(req);
> + return ret;
> +}
This logic looks almost identical to the logic in
__fuse_simple_request(). Do you think it makes sense to just call into
__fuse_simple_request() here?
> +
> ssize_t __fuse_simple_request(struct mnt_idmap *idmap,
> struct fuse_mount *fm,
> struct fuse_args *args)
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 7f16049387d1..86253517f59b 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -1273,6 +1273,20 @@ static inline ssize_t fuse_simple_idmap_request(struct mnt_idmap *idmap,
> int fuse_simple_background(struct fuse_mount *fm, struct fuse_args *args,
> gfp_t gfp_flags);
>
> +/**
> + * Compound request API
> + */
> +struct fuse_compound_req;
> +ssize_t fuse_compound_request(struct fuse_mount *fm, struct fuse_args *args);
> +ssize_t fuse_compound_send(struct fuse_compound_req *compound);
> +
> +struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, uint32_t flags);
> +int fuse_compound_add(struct fuse_compound_req *compound,
> + struct fuse_args *args);
Looking at how this is used in the 2nd patch (open+getattr), I see
that the args are first defined / filled out and then
fuse_compound_add() copies them into the compound request's buffer.
Instead of having a fuse_compound_add() api that takkes in filled out
args, what are your thoughts on having some api that returns back a
pointer to the compound buffer's current position, and then the caller
can just directly fill that out? I think that ends up saving a memcpy.
> +int fuse_compound_get_error(struct fuse_compound_req *compound,
> + int op_idx);
> +void fuse_compound_free(struct fuse_compound_req *compound);
> +
I think an alternative way of doing compound requests, though it would
only work on io-uring, is leveraging the io-uring multishot
functionality. This allows multiple cqes (requests) to be forwarded
together in one system call. Instead of batching X requests into one
fuse compound request, I think there would just be X cqes and the
normal flow of execution would remain pretty much the same (though on
the kernel side there would have to be a head "master request" that
request_wait_answer() operates on). Not sure in actuality if this
would be more complicated or simpler, but I think your approach here
probably makes more sense since it also works for /dev/fuse.
Thanks,
Joanne
> /**
> * Assign a unique id to a fuse request
> */
On Tue, Jan 06, 2026 at 05:40:52PM -0800, Joanne Koong wrote:
> On Tue, Dec 23, 2025 at 2:13 PM Horst Birthelmer
> <hbirthelmer@googlemail.com> wrote:
> >
> > For a FUSE_COMPOUND we add a header that contains information
> > about how many commands there are in the compound and about the
> > size of the expected result. This will make the interpretation
> > in libfuse easier, since we can preallocate the whole result.
> > Then we append the requests that belong to this compound.
> >
> > The API for the compound command has:
> > fuse_compound_alloc()
> > fuse_compound_add()
> > fuse_compound_request()
> > fuse_compound_free()
> >
...
> > +
> > + if (compound->buffer_pos + needed_size > compound->buffer_size) {
> > + size_t new_size = max(compound->buffer_size * 2,
> > + compound->buffer_pos + needed_size);
> > + char *new_buffer;
> > +
> > + new_size = round_up(new_size, PAGE_SIZE);
> > + new_buffer = kvrealloc(compound->buffer, new_size,
> > + GFP_KERNEL);
> > + if (!new_buffer)
> > + return -ENOMEM;
> > + compound->buffer = new_buffer;
> > + compound->buffer_size = new_size;
>
> Hmm... when we're setting up a compound request, we already know the
> size that will be needed to hold all the requests, right? Do you think
> it makes sense to allocate that from the get-go in
> fuse_compound_alloc() and then not have to do any buffer reallocation?
> I think that also gets rid of fuse_compound_req->total_size, as that
> would just be the same as fuse_compound_req->buffer_size.
>
After looking at this again, I realized it would be more efficient to not do any allocation
in fuse_compound_alloc() at all except for the fuse_compound_req, of course, and then
do all the work in fuse_compound_send().
We keep pointers to the fuse_args given to the compound command anyway since we need
to fill out the result, so why not keep just the fuse args and don't copy anything
except when actually sending it out?
I will test this version a bit and make a simplified v3.
...
>
> Thanks,
> Joanne
Thanks,
Horst
On Wed, Jan 7, 2026 at 6:03 AM Horst Birthelmer <horst@birthelmer.de> wrote:
>
> On Tue, Jan 06, 2026 at 05:40:52PM -0800, Joanne Koong wrote:
> > On Tue, Dec 23, 2025 at 2:13 PM Horst Birthelmer
> > <hbirthelmer@googlemail.com> wrote:
> > >
> > > For a FUSE_COMPOUND we add a header that contains information
> > > about how many commands there are in the compound and about the
> > > size of the expected result. This will make the interpretation
> > > in libfuse easier, since we can preallocate the whole result.
> > > Then we append the requests that belong to this compound.
> > >
> > > The API for the compound command has:
> > > fuse_compound_alloc()
> > > fuse_compound_add()
> > > fuse_compound_request()
> > > fuse_compound_free()
> > >
> ...
> > > +
> > > + if (compound->buffer_pos + needed_size > compound->buffer_size) {
> > > + size_t new_size = max(compound->buffer_size * 2,
> > > + compound->buffer_pos + needed_size);
> > > + char *new_buffer;
> > > +
> > > + new_size = round_up(new_size, PAGE_SIZE);
> > > + new_buffer = kvrealloc(compound->buffer, new_size,
> > > + GFP_KERNEL);
> > > + if (!new_buffer)
> > > + return -ENOMEM;
> > > + compound->buffer = new_buffer;
> > > + compound->buffer_size = new_size;
> >
> > Hmm... when we're setting up a compound request, we already know the
> > size that will be needed to hold all the requests, right? Do you think
> > it makes sense to allocate that from the get-go in
> > fuse_compound_alloc() and then not have to do any buffer reallocation?
> > I think that also gets rid of fuse_compound_req->total_size, as that
> > would just be the same as fuse_compound_req->buffer_size.
> >
> After looking at this again, I realized it would be more efficient to not do any allocation
> in fuse_compound_alloc() at all except for the fuse_compound_req, of course, and then
> do all the work in fuse_compound_send().
> We keep pointers to the fuse_args given to the compound command anyway since we need
> to fill out the result, so why not keep just the fuse args and don't copy anything
> except when actually sending it out?
>
> I will test this version a bit and make a simplified v3.
Awesome, looking forward to seeing v3.
Thanks,
Joanne
>
> ...
> >
> > Thanks,
> > Joanne
>
> Thanks,
> Horst
Hi Joanne,
first ... thank you so much for taking the time.
On Tue, Jan 06, 2026 at 05:40:52PM -0800, Joanne Koong wrote:
> On Tue, Dec 23, 2025 at 2:13 PM Horst Birthelmer
> <hbirthelmer@googlemail.com> wrote:
> >
> > For a FUSE_COMPOUND we add a header that contains information
> > about how many commands there are in the compound and about the
> > size of the expected result. This will make the interpretation
> > in libfuse easier, since we can preallocate the whole result.
> > Then we append the requests that belong to this compound.
> >
> > The API for the compound command has:
> > fuse_compound_alloc()
> > fuse_compound_add()
> > fuse_compound_request()
> > fuse_compound_free()
> >
...
> > +
> > + if (offset != compound->buffer_pos) {
> > + pr_info_ratelimited("FUSE: compound buffer size mismatch (calculated %zu bytes, actual %zu bytes)\n",
> > + offset, compound->buffer_pos);
> > + return -EINVAL;
> > + }
> > +
> > + return 0;
>
> I wonder if this is overkill to have given that the kernel is in
> charge of setting up the compound request and if the code is right,
> has done it correctly. imo it adds extra overhead but doesn't offer
> much given that the kernel code should aalready be correct.
You are completely right. It was just a big help during development and has to be taken out eventually.
I don't really like #ifdefs very much. Do you think we should throw it out completely or just not use it in the usual code path?
> > +}
> > +
> > +int fuse_compound_add(struct fuse_compound_req *compound,
> > + struct fuse_args *args)
> > +{
> > + struct fuse_in_header *hdr;
> > + size_t args_size = 0;
> > + size_t needed_size;
> > + size_t expected_out_size = 0;
> > + int i;
> > +
> > + if (!compound ||
> > + compound->compound_header.count >= FUSE_MAX_COMPOUND_OPS)
> > + return -EINVAL;
> > +
> > + if (args->in_pages)
> > + return -EINVAL;
> > +
> > + for (i = 0; i < args->in_numargs; i++)
> > + args_size += args->in_args[i].size;
> > +
> > + for (i = 0; i < args->out_numargs; i++)
> > + expected_out_size += args->out_args[i].size;
> > +
> > + needed_size = sizeof(struct fuse_in_header) + args_size;
> > +
> > + if (compound->buffer_pos + needed_size > compound->buffer_size) {
> > + size_t new_size = max(compound->buffer_size * 2,
> > + compound->buffer_pos + needed_size);
> > + char *new_buffer;
> > +
> > + new_size = round_up(new_size, PAGE_SIZE);
> > + new_buffer = kvrealloc(compound->buffer, new_size,
> > + GFP_KERNEL);
> > + if (!new_buffer)
> > + return -ENOMEM;
> > + compound->buffer = new_buffer;
> > + compound->buffer_size = new_size;
>
> Hmm... when we're setting up a compound request, we already know the
> size that will be needed to hold all the requests, right? Do you think
> it makes sense to allocate that from the get-go in
> fuse_compound_alloc() and then not have to do any buffer reallocation?
> I think that also gets rid of fuse_compound_req->total_size, as that
> would just be the same as fuse_compound_req->buffer_size.
We could do that. But then we would have to have the whole picture from the start.
What I'm trying to say is (and at the moment there is no example to the contrary)
that we could not create a compound without exactly knowing what will be in it.
So by the time we do the allocation the calls should all be there.
I have no counter argument for that at the moment which doesn't mean there isn't one.
The API felt more inuitive to me doing alloc, then add, then send, but it would be faster
the other way around.
>
> > + }
> > +
> > + /* Build request header */
> > + hdr = (struct fuse_in_header *)(compound->buffer +
> > + compound->buffer_pos);
> > + memset(hdr, 0, sizeof(*hdr));
> > + hdr->len = needed_size;
> > + hdr->opcode = args->opcode;
> > + hdr->nodeid = args->nodeid;
> > + hdr->uid = from_kuid(compound->fm->fc->user_ns, current_fsuid());
> > + hdr->gid = from_kgid(compound->fm->fc->user_ns, current_fsgid());
> > + hdr->pid = pid_nr_ns(task_pid(current), compound->fm->fc->pid_ns);
> > + hdr->unique = fuse_get_unique(&compound->fm->fc->iq);
> > + compound->buffer_pos += sizeof(*hdr);
> > +
> > + for (i = 0; i < args->in_numargs; i++) {
> > + memcpy(compound->buffer + compound->buffer_pos,
> > + args->in_args[i].value, args->in_args[i].size);
> > + compound->buffer_pos += args->in_args[i].size;
> > + }
> > +
> > + compound->total_expected_out_size += expected_out_size;
> > +
> > + /* Store args for response parsing */
> > + compound->op_args[compound->compound_header.count] = args;
> > +
> > + compound->compound_header.count++;
> > + compound->total_size += needed_size;
> > +
> > + return 0;
>
> Does fc->max_pages need to be accounted for as well? iirc, that's the
> upper limit on how many pages can be forwarded to the server in a
> request.
>
Eventually yes. For open+getattr and lookup+create I didn't think it was relevant.
Am I missing something obvious?
...
> > +
> > + fuse_args_to_req(req, args);
> > +
> > + if (!args->noreply)
> > + __set_bit(FR_ISREPLY, &req->flags);
> > +
> > + __fuse_request_send(req);
> > + ret = req->out.h.error;
> > + if (!ret && args->out_argvar) {
> > + BUG_ON(args->out_numargs == 0);
> > + ret = args->out_args[args->out_numargs - 1].size;
> > + }
> > + fuse_put_request(req);
> > + return ret;
> > +}
>
> This logic looks almost identical to the logic in
> __fuse_simple_request(). Do you think it makes sense to just call into
> __fuse_simple_request() here?
When I wrote it, I thought I had to do a different error handling.
After testing I realized, that I could actually treat the compound
as a normal request.
So, yes I think so.
Will be changed in v3.
>
> > +
> > ssize_t __fuse_simple_request(struct mnt_idmap *idmap,
> > struct fuse_mount *fm,
> > struct fuse_args *args)
> > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > index 7f16049387d1..86253517f59b 100644
> > --- a/fs/fuse/fuse_i.h
> > +++ b/fs/fuse/fuse_i.h
> > @@ -1273,6 +1273,20 @@ static inline ssize_t fuse_simple_idmap_request(struct mnt_idmap *idmap,
> > int fuse_simple_background(struct fuse_mount *fm, struct fuse_args *args,
> > gfp_t gfp_flags);
> >
> > +/**
> > + * Compound request API
> > + */
> > +struct fuse_compound_req;
> > +ssize_t fuse_compound_request(struct fuse_mount *fm, struct fuse_args *args);
> > +ssize_t fuse_compound_send(struct fuse_compound_req *compound);
> > +
> > +struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, uint32_t flags);
> > +int fuse_compound_add(struct fuse_compound_req *compound,
> > + struct fuse_args *args);
>
> Looking at how this is used in the 2nd patch (open+getattr), I see
> that the args are first defined / filled out and then
> fuse_compound_add() copies them into the compound request's buffer.
> Instead of having a fuse_compound_add() api that takkes in filled out
> args, what are your thoughts on having some api that returns back a
> pointer to the compound buffer's current position, and then the caller
> can just directly fill that out? I think that ends up saving a memcpy.
>
I have to think about that.
Sounds like a good point, but makes the api slightly more complicated.
> > +int fuse_compound_get_error(struct fuse_compound_req *compound,
> > + int op_idx);
> > +void fuse_compound_free(struct fuse_compound_req *compound);
> > +
>
> I think an alternative way of doing compound requests, though it would
> only work on io-uring, is leveraging the io-uring multishot
> functionality. This allows multiple cqes (requests) to be forwarded
> together in one system call. Instead of batching X requests into one
> fuse compound request, I think there would just be X cqes and the
> normal flow of execution would remain pretty much the same (though on
> the kernel side there would have to be a head "master request" that
> request_wait_answer() operates on). Not sure in actuality if this
> would be more complicated or simpler, but I think your approach here
> probably makes more sense since it also works for /dev/fuse.
>
I had a similar discussion with Bernd for libfuse.
I completely get your point, but I think it would really complicate things.
I would really want to keep this as simple as I can, since the handling
gets complicated very fast the moment we get more calls and have to handle
pages.
Bernd already had some idea about doing writes while using compounds making the
handling different form /dev/fuse to io-uring was too scary for me.
> Thanks,
> Joanne
>
Thanks,
Horst
On Wed, Jan 7, 2026 at 1:29 AM Horst Birthelmer <horst@birthelmer.de> wrote:
>
Hi Horst,
>
> Hi Joanne,
>
> first ... thank you so much for taking the time.
>
> On Tue, Jan 06, 2026 at 05:40:52PM -0800, Joanne Koong wrote:
> > On Tue, Dec 23, 2025 at 2:13 PM Horst Birthelmer
> > <hbirthelmer@googlemail.com> wrote:
> > >
> > > For a FUSE_COMPOUND we add a header that contains information
> > > about how many commands there are in the compound and about the
> > > size of the expected result. This will make the interpretation
> > > in libfuse easier, since we can preallocate the whole result.
> > > Then we append the requests that belong to this compound.
> > >
> > > The API for the compound command has:
> > > fuse_compound_alloc()
> > > fuse_compound_add()
> > > fuse_compound_request()
> > > fuse_compound_free()
> > >
> ...
> > > +
> > > + if (offset != compound->buffer_pos) {
> > > + pr_info_ratelimited("FUSE: compound buffer size mismatch (calculated %zu bytes, actual %zu bytes)\n",
> > > + offset, compound->buffer_pos);
> > > + return -EINVAL;
> > > + }
> > > +
> > > + return 0;
> >
> > I wonder if this is overkill to have given that the kernel is in
> > charge of setting up the compound request and if the code is right,
> > has done it correctly. imo it adds extra overhead but doesn't offer
> > much given that the kernel code should aalready be correct.
>
> You are completely right. It was just a big help during development and has to be taken out eventually.
> I don't really like #ifdefs very much. Do you think we should throw it out completely or just not use it in the usual code path?
>
imo I think we should just get rid of it entirely.
> > > +}
> > > +
> > > +int fuse_compound_add(struct fuse_compound_req *compound,
> > > + struct fuse_args *args)
> > > +{
> > > + struct fuse_in_header *hdr;
> > > + size_t args_size = 0;
> > > + size_t needed_size;
> > > + size_t expected_out_size = 0;
> > > + int i;
> > > +
> > > + if (!compound ||
> > > + compound->compound_header.count >= FUSE_MAX_COMPOUND_OPS)
> > > + return -EINVAL;
> > > +
> > > + if (args->in_pages)
> > > + return -EINVAL;
> > > +
> > > + for (i = 0; i < args->in_numargs; i++)
> > > + args_size += args->in_args[i].size;
> > > +
> > > + for (i = 0; i < args->out_numargs; i++)
> > > + expected_out_size += args->out_args[i].size;
> > > +
> > > + needed_size = sizeof(struct fuse_in_header) + args_size;
> > > +
> > > + if (compound->buffer_pos + needed_size > compound->buffer_size) {
> > > + size_t new_size = max(compound->buffer_size * 2,
> > > + compound->buffer_pos + needed_size);
> > > + char *new_buffer;
> > > +
> > > + new_size = round_up(new_size, PAGE_SIZE);
> > > + new_buffer = kvrealloc(compound->buffer, new_size,
> > > + GFP_KERNEL);
> > > + if (!new_buffer)
> > > + return -ENOMEM;
> > > + compound->buffer = new_buffer;
> > > + compound->buffer_size = new_size;
> >
> > Hmm... when we're setting up a compound request, we already know the
> > size that will be needed to hold all the requests, right? Do you think
> > it makes sense to allocate that from the get-go in
> > fuse_compound_alloc() and then not have to do any buffer reallocation?
> > I think that also gets rid of fuse_compound_req->total_size, as that
> > would just be the same as fuse_compound_req->buffer_size.
>
> We could do that. But then we would have to have the whole picture from the start.
> What I'm trying to say is (and at the moment there is no example to the contrary)
> that we could not create a compound without exactly knowing what will be in it.
>
> So by the time we do the allocation the calls should all be there.
> I have no counter argument for that at the moment which doesn't mean there isn't one.
> The API felt more inuitive to me doing alloc, then add, then send, but it would be faster
> the other way around.
>
> >
> > > + }
> > > +
> > > + /* Build request header */
> > > + hdr = (struct fuse_in_header *)(compound->buffer +
> > > + compound->buffer_pos);
> > > + memset(hdr, 0, sizeof(*hdr));
> > > + hdr->len = needed_size;
> > > + hdr->opcode = args->opcode;
> > > + hdr->nodeid = args->nodeid;
> > > + hdr->uid = from_kuid(compound->fm->fc->user_ns, current_fsuid());
> > > + hdr->gid = from_kgid(compound->fm->fc->user_ns, current_fsgid());
> > > + hdr->pid = pid_nr_ns(task_pid(current), compound->fm->fc->pid_ns);
> > > + hdr->unique = fuse_get_unique(&compound->fm->fc->iq);
> > > + compound->buffer_pos += sizeof(*hdr);
> > > +
> > > + for (i = 0; i < args->in_numargs; i++) {
> > > + memcpy(compound->buffer + compound->buffer_pos,
> > > + args->in_args[i].value, args->in_args[i].size);
> > > + compound->buffer_pos += args->in_args[i].size;
> > > + }
> > > +
> > > + compound->total_expected_out_size += expected_out_size;
> > > +
> > > + /* Store args for response parsing */
> > > + compound->op_args[compound->compound_header.count] = args;
> > > +
> > > + compound->compound_header.count++;
> > > + compound->total_size += needed_size;
> > > +
> > > + return 0;
> >
> > Does fc->max_pages need to be accounted for as well? iirc, that's the
> > upper limit on how many pages can be forwarded to the server in a
> > request.
> >
> Eventually yes. For open+getattr and lookup+create I didn't think it was relevant.
> Am I missing something obvious?
I don't think it's relevant for open+getattr and lookup+create either
since they won't exceed PAGE_SIZE, but imo I think it'd be good to
have the fc->max_pages logic added here since the api is a general
api.
>
> ...
> > > +
> > > + fuse_args_to_req(req, args);
> > > +
> > > + if (!args->noreply)
> > > + __set_bit(FR_ISREPLY, &req->flags);
> > > +
> > > + __fuse_request_send(req);
> > > + ret = req->out.h.error;
> > > + if (!ret && args->out_argvar) {
> > > + BUG_ON(args->out_numargs == 0);
> > > + ret = args->out_args[args->out_numargs - 1].size;
> > > + }
> > > + fuse_put_request(req);
> > > + return ret;
> > > +}
> >
> > This logic looks almost identical to the logic in
> > __fuse_simple_request(). Do you think it makes sense to just call into
> > __fuse_simple_request() here?
>
> When I wrote it, I thought I had to do a different error handling.
> After testing I realized, that I could actually treat the compound
> as a normal request.
> So, yes I think so.
> Will be changed in v3.
>
> >
> > > +
> > > ssize_t __fuse_simple_request(struct mnt_idmap *idmap,
> > > struct fuse_mount *fm,
> > > struct fuse_args *args)
> > > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > > index 7f16049387d1..86253517f59b 100644
> > > --- a/fs/fuse/fuse_i.h
> > > +++ b/fs/fuse/fuse_i.h
> > > @@ -1273,6 +1273,20 @@ static inline ssize_t fuse_simple_idmap_request(struct mnt_idmap *idmap,
> > > int fuse_simple_background(struct fuse_mount *fm, struct fuse_args *args,
> > > gfp_t gfp_flags);
> > >
> > > +/**
> > > + * Compound request API
> > > + */
> > > +struct fuse_compound_req;
> > > +ssize_t fuse_compound_request(struct fuse_mount *fm, struct fuse_args *args);
> > > +ssize_t fuse_compound_send(struct fuse_compound_req *compound);
> > > +
> > > +struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, uint32_t flags);
> > > +int fuse_compound_add(struct fuse_compound_req *compound,
> > > + struct fuse_args *args);
> >
> > Looking at how this is used in the 2nd patch (open+getattr), I see
> > that the args are first defined / filled out and then
> > fuse_compound_add() copies them into the compound request's buffer.
> > Instead of having a fuse_compound_add() api that takkes in filled out
> > args, what are your thoughts on having some api that returns back a
> > pointer to the compound buffer's current position, and then the caller
> > can just directly fill that out? I think that ends up saving a memcpy.
> >
> I have to think about that.
> Sounds like a good point, but makes the api slightly more complicated.
>
> > > +int fuse_compound_get_error(struct fuse_compound_req *compound,
> > > + int op_idx);
> > > +void fuse_compound_free(struct fuse_compound_req *compound);
> > > +
> >
> > I think an alternative way of doing compound requests, though it would
> > only work on io-uring, is leveraging the io-uring multishot
> > functionality. This allows multiple cqes (requests) to be forwarded
> > together in one system call. Instead of batching X requests into one
> > fuse compound request, I think there would just be X cqes and the
> > normal flow of execution would remain pretty much the same (though on
> > the kernel side there would have to be a head "master request" that
> > request_wait_answer() operates on). Not sure in actuality if this
> > would be more complicated or simpler, but I think your approach here
> > probably makes more sense since it also works for /dev/fuse.
> >
> I had a similar discussion with Bernd for libfuse.
> I completely get your point, but I think it would really complicate things.
>
> I would really want to keep this as simple as I can, since the handling
> gets complicated very fast the moment we get more calls and have to handle
> pages.
>
> Bernd already had some idea about doing writes while using compounds making the
> handling different form /dev/fuse to io-uring was too scary for me.
That makes sense, I think this approach overall is simpler than the
io-uring approach and I agree that simpler is better :) I think for
some compound requests though, it would only suffice with the io-uring
approach due to the inherent limitation of /dev/fuse being bounded by
the size of the lone buffer that's used for reads/writes to the
/dev/fuse fd, whereas with fuse io-uring, the queue is set up with the
capacity to service multiple big buffers in parallel per 1 system
call. This only comes into play, I think, for compounding readahead
and writeback requests. Eg the buffer used for /dev/fuse is usually 1
MB, which means the upper limit is writing back 1 MB per system call,
but with io-uring, the default queue depth is 8 which means we're
potentially able to send 8 MB of writeback data per 1 system call. I
think it'd be very nice if we could reduce the number of system calls
needed for readahead/writeback. I think maybe this could be orthogonal
to the compound requests implementation you have in this patchset (eg
for readahead/writeback, we use io-uring multishot to compound those
requests together to be sent in 1 system call, and for other requests
like open+getattr, we use the compound request infrastructure you've
added here), since some servers may not be using io-uring and overall,
I think implementing io-uring multishot for fuse requests is a lot
simpler if those requests are all background requests (which they are
for readahead/writeback requests).
Thanks,
Joanne
>
> > Thanks,
> > Joanne
> >
>
> Thanks,
> Horst
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