Introduce concept of "Live Update Sessions" within the LUO framework.
LUO sessions provide a mechanism to group and manage `struct file *`
instances (representing file descriptors) that need to be preserved
across a kexec-based live update.

Each session is identified by a unique name and acts as a container
for file objects whose state is critical to a userspace workload, such
as a virtual machine or a high-performance database, aiming to maintain
their functionality across a kernel transition.

This groundwork establishes the framework for preserving file-backed
state across kernel updates, with the actual file data preservation
mechanisms to be implemented in subsequent patches.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/kho/abi/luo.h      |  71 +++++
 include/uapi/linux/liveupdate.h  |   3 +
 kernel/liveupdate/Makefile       |   3 +-
 kernel/liveupdate/luo_core.c     |   9 +
 kernel/liveupdate/luo_internal.h |  29 ++
 kernel/liveupdate/luo_session.c  | 462 +++++++++++++++++++++++++++++++
 6 files changed, 576 insertions(+), 1 deletion(-)
 create mode 100644 kernel/liveupdate/luo_session.c

diff --git a/include/linux/kho/abi/luo.h b/include/linux/kho/abi/luo.h
index 8523b3ff82d1..a2d2940eca6b 100644
--- a/include/linux/kho/abi/luo.h
+++ b/include/linux/kho/abi/luo.h
@@ -28,6 +28,11 @@
  *     / {
  *         compatible = "luo-v1";
  *         liveupdate-number = <...>;
+ *
+ *         luo-session {
+ *             compatible = "luo-session-v1";
+ *             luo-session-header = <phys_addr_of_session_header_ser>;
+ *         };
  *     };
  *
  * Main LUO Node (/):
@@ -36,11 +41,37 @@
  *     Identifies the overall LUO ABI version.
  *   - liveupdate-number: u64
  *     A counter tracking the number of successful live updates performed.
+ *
+ * Session Node (luo-session):
+ *   This node describes all preserved user-space sessions.
+ *
+ *   - compatible: "luo-session-v1"
+ *     Identifies the session ABI version.
+ *   - luo-session-header: u64
+ *     The physical address of a `struct luo_session_header_ser`. This structure
+ *     is the header for a contiguous block of memory containing an array of
+ *     `struct luo_session_ser`, one for each preserved session.
+ *
+ * Serialization Structures:
+ *   The FDT properties point to memory regions containing arrays of simple,
+ *   `__packed` structures. These structures contain the actual preserved state.
+ *
+ *   - struct luo_session_header_ser:
+ *     Header for the session array. Contains the total page count of the
+ *     preserved memory block and the number of `struct luo_session_ser`
+ *     entries that follow.
+ *
+ *   - struct luo_session_ser:
+ *     Metadata for a single session, including its name and a physical pointer
+ *     to another preserved memory block containing an array of
+ *     `struct luo_file_ser` for all files in that session.
  */
 
 #ifndef _LINUX_KHO_ABI_LUO_H
 #define _LINUX_KHO_ABI_LUO_H
 
+#include <uapi/linux/liveupdate.h>
+
 /*
  * The LUO FDT hooks all LUO state for sessions, fds, etc.
  * In the root it also carries "liveupdate-number" 64-bit property that
@@ -51,4 +82,44 @@
 #define LUO_FDT_COMPATIBLE	"luo-v1"
 #define LUO_FDT_LIVEUPDATE_NUM	"liveupdate-number"
 
+/*
+ * LUO FDT session node
+ * LUO_FDT_SESSION_HEADER:  is a u64 physical address of struct
+ *                          luo_session_header_ser
+ */
+#define LUO_FDT_SESSION_NODE_NAME	"luo-session"
+#define LUO_FDT_SESSION_COMPATIBLE	"luo-session-v1"
+#define LUO_FDT_SESSION_HEADER		"luo-session-header"
+
+/**
+ * struct luo_session_header_ser - Header for the serialized session data block.
+ * @count: The number of `struct luo_session_ser` entries that immediately
+ *         follow this header in the memory block.
+ *
+ * This structure is located at the beginning of a contiguous block of
+ * physical memory preserved across the kexec. It provides the necessary
+ * metadata to interpret the array of session entries that follow.
+ *
+ * If this structure is modified, `LUO_FDT_SESSION_COMPATIBLE` must be updated.
+ */
+struct luo_session_header_ser {
+	u64 count;
+} __packed;
+
+/**
+ * struct luo_session_ser - Represents the serialized metadata for a LUO session.
+ * @name:         The unique name of the session, provided by the userspace at
+ *                the time of session creation.
+ *
+ * This structure is used to package session-specific metadata for transfer
+ * between kernels via Kexec Handover. An array of these structures (one per
+ * session) is created and passed to the new kernel, allowing it to reconstruct
+ * the session context.
+ *
+ * If this structure is modified, `LUO_FDT_SESSION_COMPATIBLE` must be updated.
+ */
+struct luo_session_ser {
+	char name[LIVEUPDATE_SESSION_NAME_LENGTH];
+} __packed;
+
 #endif /* _LINUX_KHO_ABI_LUO_H */
diff --git a/include/uapi/linux/liveupdate.h b/include/uapi/linux/liveupdate.h
index df34c1642c4d..40578ae19668 100644
--- a/include/uapi/linux/liveupdate.h
+++ b/include/uapi/linux/liveupdate.h
@@ -43,4 +43,7 @@
 /* The ioctl type, documented in ioctl-number.rst */
 #define LIVEUPDATE_IOCTL_TYPE		0xBA
 
+/* The maximum length of session name including null termination */
+#define LIVEUPDATE_SESSION_NAME_LENGTH 64
+
 #endif /* _UAPI_LIVEUPDATE_H */
diff --git a/kernel/liveupdate/Makefile b/kernel/liveupdate/Makefile
index 08954c1770c4..6af93caa58cf 100644
--- a/kernel/liveupdate/Makefile
+++ b/kernel/liveupdate/Makefile
@@ -1,7 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0
 
 luo-y :=								\
-		luo_core.o
+		luo_core.o						\
+		luo_session.o
 
 obj-$(CONFIG_KEXEC_HANDOVER)		+= kexec_handover.o
 obj-$(CONFIG_KEXEC_HANDOVER_DEBUG)	+= kexec_handover_debug.o
diff --git a/kernel/liveupdate/luo_core.c b/kernel/liveupdate/luo_core.c
index 9f9fe9a81b29..a0f7788cd003 100644
--- a/kernel/liveupdate/luo_core.c
+++ b/kernel/liveupdate/luo_core.c
@@ -118,6 +118,10 @@ static int __init luo_early_startup(void)
 	pr_info("Retrieved live update data, liveupdate number: %lld\n",
 		luo_global.liveupdate_num);
 
+	err = luo_session_setup_incoming(luo_global.fdt_in);
+	if (err)
+		return err;
+
 	return 0;
 }
 
@@ -154,6 +158,7 @@ static int __init luo_fdt_setup(void)
 	err |= fdt_begin_node(fdt_out, "");
 	err |= fdt_property_string(fdt_out, "compatible", LUO_FDT_COMPATIBLE);
 	err |= fdt_property(fdt_out, LUO_FDT_LIVEUPDATE_NUM, &ln, sizeof(ln));
+	err |= luo_session_setup_outgoing(fdt_out);
 	err |= fdt_end_node(fdt_out);
 	err |= fdt_finish(fdt_out);
 	if (err)
@@ -211,6 +216,10 @@ int liveupdate_reboot(void)
 	if (!liveupdate_enabled())
 		return 0;
 
+	err = luo_session_serialize();
+	if (err)
+		return err;
+
 	err = kho_finalize();
 	if (err) {
 		pr_err("kho_finalize failed %d\n", err);
diff --git a/kernel/liveupdate/luo_internal.h b/kernel/liveupdate/luo_internal.h
index 8612687b2000..05ae91695ec6 100644
--- a/kernel/liveupdate/luo_internal.h
+++ b/kernel/liveupdate/luo_internal.h
@@ -19,4 +19,33 @@
  */
 #define luo_restore_fail(__fmt, ...) panic(__fmt, ##__VA_ARGS__)
 
+/**
+ * struct luo_session - Represents an active or incoming Live Update session.
+ * @name:       A unique name for this session, used for identification and
+ *              retrieval.
+ * @ser:        Pointer to the serialized data for this session.
+ * @list:       A list_head member used to link this session into a global list
+ *              of either outgoing (to be preserved) or incoming (restored from
+ *              previous kernel) sessions.
+ * @retrieved:  A boolean flag indicating whether this session has been
+ *              retrieved by a consumer in the new kernel.
+ * @mutex:      protects fields in the luo_session.
+ */
+struct luo_session {
+	char name[LIVEUPDATE_SESSION_NAME_LENGTH];
+	struct luo_session_ser *ser;
+	struct list_head list;
+	bool retrieved;
+	struct mutex mutex;
+};
+
+int luo_session_create(const char *name, struct file **filep);
+int luo_session_retrieve(const char *name, struct file **filep);
+int __init luo_session_setup_outgoing(void *fdt);
+int __init luo_session_setup_incoming(void *fdt);
+int luo_session_serialize(void);
+int luo_session_deserialize(void);
+bool luo_session_quiesce(void);
+void luo_session_resume(void);
+
 #endif /* _LINUX_LUO_INTERNAL_H */
diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
new file mode 100644
index 000000000000..ee363d1b2b10
--- /dev/null
+++ b/kernel/liveupdate/luo_session.c
@@ -0,0 +1,462 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (c) 2025, Google LLC.
+ * Pasha Tatashin <pasha.tatashin@soleen.com>
+ */
+
+/**
+ * DOC: LUO Sessions
+ *
+ * LUO Sessions provide the core mechanism for grouping and managing `struct
+ * file *` instances that need to be preserved across a kexec-based live
+ * update. Each session acts as a named container for a set of file objects,
+ * allowing a userspace agent to manage the lifecycle of resources critical to a
+ * workload.
+ *
+ * Core Concepts:
+ *
+ * - Named Containers: Sessions are identified by a unique, user-provided name,
+ *   which is used for both creation in the current kernel and retrieval in the
+ *   next kernel.
+ *
+ * - Userspace Interface: Session management is driven from userspace via
+ *   ioctls on /dev/liveupdate.
+ *
+ * - Serialization: Session metadata is preserved using the KHO framework. When
+ *   a live update is triggered via kexec, an array of `struct luo_session_ser`
+ *   is populated and placed in a preserved memory region. An FDT node is also
+ *   created, containing the count of sessions and the physical address of this
+ *   array.
+ *
+ * Session Lifecycle:
+ *
+ * 1.  Creation: A userspace agent calls `luo_session_create()` to create a
+ *     new, empty session and receives a file descriptor for it.
+ *
+ * 2.  Serialization: When the `reboot(LINUX_REBOOT_CMD_KEXEC)` syscall is
+ *     made, `luo_session_serialize()` is called. It iterates through all
+ *     active sessions and writes their metadata into a memory area preserved
+ *     by KHO.
+ *
+ * 3.  Deserialization (in new kernel): After kexec, `luo_session_deserialize()`
+ *     runs, reading the serialized data and creating a list of `struct
+ *     luo_session` objects representing the preserved sessions.
+ *
+ * 4.  Retrieval: A userspace agent in the new kernel can then call
+ *     `luo_session_retrieve()` with a session name to get a new file
+ *     descriptor and access the preserved state.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/anon_inodes.h>
+#include <linux/cleanup.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/io.h>
+#include <linux/kexec_handover.h>
+#include <linux/kho/abi/luo.h>
+#include <linux/libfdt.h>
+#include <linux/list.h>
+#include <linux/liveupdate.h>
+#include <linux/mutex.h>
+#include <linux/rwsem.h>
+#include <linux/slab.h>
+#include <linux/unaligned.h>
+#include <uapi/linux/liveupdate.h>
+#include "luo_internal.h"
+
+/* 16 4K pages, give space for 744 sessions */
+#define LUO_SESSION_PGCNT	16ul
+#define LUO_SESSION_MAX		(((LUO_SESSION_PGCNT << PAGE_SHIFT) -	\
+		sizeof(struct luo_session_header_ser)) /		\
+		sizeof(struct luo_session_ser))
+
+/**
+ * struct luo_session_header - Header struct for managing LUO sessions.
+ * @count:      The number of sessions currently tracked in the @list.
+ * @list:       The head of the linked list of `struct luo_session` instances.
+ * @rwsem:      A read-write semaphore providing synchronized access to the
+ *              session list and other fields in this structure.
+ * @header_ser: The header data of serialization array.
+ * @ser:        The serialized session data (an array of
+ *              `struct luo_session_ser`).
+ * @active:     Set to true when first initialized. If previous kernel did not
+ *              send session data, active stays false for incoming.
+ */
+struct luo_session_header {
+	long count;
+	struct list_head list;
+	struct rw_semaphore rwsem;
+	struct luo_session_header_ser *header_ser;
+	struct luo_session_ser *ser;
+	bool active;
+};
+
+/**
+ * struct luo_session_global - Global container for managing LUO sessions.
+ * @incoming:     The sessions passed from the previous kernel.
+ * @outgoing:     The sessions that are going to be passed to the next kernel.
+ */
+struct luo_session_global {
+	struct luo_session_header incoming;
+	struct luo_session_header outgoing;
+};
+
+static struct luo_session_global luo_session_global = {
+	.incoming = {
+		.list = LIST_HEAD_INIT(luo_session_global.incoming.list),
+		.rwsem = __RWSEM_INITIALIZER(luo_session_global.incoming.rwsem),
+	},
+	.outgoing = {
+		.list = LIST_HEAD_INIT(luo_session_global.outgoing.list),
+		.rwsem = __RWSEM_INITIALIZER(luo_session_global.outgoing.rwsem),
+	},
+};
+
+static struct luo_session *luo_session_alloc(const char *name)
+{
+	struct luo_session *session = kzalloc(sizeof(*session), GFP_KERNEL);
+
+	if (!session)
+		return ERR_PTR(-ENOMEM);
+
+	strscpy(session->name, name, sizeof(session->name));
+	INIT_LIST_HEAD(&session->list);
+	mutex_init(&session->mutex);
+
+	return session;
+}
+
+static void luo_session_free(struct luo_session *session)
+{
+	mutex_destroy(&session->mutex);
+	kfree(session);
+}
+
+static int luo_session_insert(struct luo_session_header *sh,
+			      struct luo_session *session)
+{
+	struct luo_session *it;
+
+	guard(rwsem_write)(&sh->rwsem);
+
+	/*
+	 * For outgoing we should make sure there is room in serialization array
+	 * for new session.
+	 */
+	if (sh == &luo_session_global.outgoing) {
+		if (sh->count == LUO_SESSION_MAX)
+			return -ENOMEM;
+	}
+
+	/*
+	 * For small number of sessions this loop won't hurt performance
+	 * but if we ever start using a lot of sessions, this might
+	 * become a bottle neck during deserialization time, as it would
+	 * cause O(n*n) complexity.
+	 */
+	list_for_each_entry(it, &sh->list, list) {
+		if (!strncmp(it->name, session->name, sizeof(it->name)))
+			return -EEXIST;
+	}
+	list_add_tail(&session->list, &sh->list);
+	sh->count++;
+
+	return 0;
+}
+
+static void luo_session_remove(struct luo_session_header *sh,
+			       struct luo_session *session)
+{
+	guard(rwsem_write)(&sh->rwsem);
+	list_del(&session->list);
+	sh->count--;
+}
+
+static int luo_session_release(struct inode *inodep, struct file *filep)
+{
+	struct luo_session *session = filep->private_data;
+	struct luo_session_header *sh;
+
+	/* If retrieved is set, it means this session is from incoming list */
+	if (session->retrieved)
+		sh = &luo_session_global.incoming;
+	else
+		sh = &luo_session_global.outgoing;
+
+	luo_session_remove(sh, session);
+	luo_session_free(session);
+
+	return 0;
+}
+
+static const struct file_operations luo_session_fops = {
+	.owner = THIS_MODULE,
+	.release = luo_session_release,
+};
+
+/* Create a "struct file" for session */
+static int luo_session_getfile(struct luo_session *session, struct file **filep)
+{
+	char name_buf[128];
+	struct file *file;
+
+	lockdep_assert_held(&session->mutex);
+	snprintf(name_buf, sizeof(name_buf), "[luo_session] %s", session->name);
+	file = anon_inode_getfile(name_buf, &luo_session_fops, session, O_RDWR);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	*filep = file;
+
+	return 0;
+}
+
+int luo_session_create(const char *name, struct file **filep)
+{
+	struct luo_session *session;
+	int err;
+
+	session = luo_session_alloc(name);
+	if (IS_ERR(session))
+		return PTR_ERR(session);
+
+	err = luo_session_insert(&luo_session_global.outgoing, session);
+	if (err)
+		goto err_free;
+
+	scoped_guard(mutex, &session->mutex)
+		err = luo_session_getfile(session, filep);
+	if (err)
+		goto err_remove;
+
+	return 0;
+
+err_remove:
+	luo_session_remove(&luo_session_global.outgoing, session);
+err_free:
+	luo_session_free(session);
+
+	return err;
+}
+
+int luo_session_retrieve(const char *name, struct file **filep)
+{
+	struct luo_session_header *sh = &luo_session_global.incoming;
+	struct luo_session *session = NULL;
+	struct luo_session *it;
+	int err;
+
+	scoped_guard(rwsem_read, &sh->rwsem) {
+		list_for_each_entry(it, &sh->list, list) {
+			if (!strncmp(it->name, name, sizeof(it->name))) {
+				session = it;
+				break;
+			}
+		}
+	}
+
+	if (!session)
+		return -ENOENT;
+
+	guard(mutex)(&session->mutex);
+	if (session->retrieved)
+		return -EINVAL;
+
+	err = luo_session_getfile(session, filep);
+	if (!err)
+		session->retrieved = true;
+
+	return err;
+}
+
+int __init luo_session_setup_outgoing(void *fdt_out)
+{
+	struct luo_session_header_ser *header_ser;
+	void *outgoing_buffer;
+	u64 header_ser_pa;
+	int err;
+
+	outgoing_buffer = kho_alloc_preserve(LUO_SESSION_PGCNT << PAGE_SHIFT);
+	if (IS_ERR(outgoing_buffer))
+		return PTR_ERR(header_ser);
+	header_ser = outgoing_buffer;
+	header_ser_pa = virt_to_phys(header_ser);
+
+	err = fdt_begin_node(fdt_out, LUO_FDT_SESSION_NODE_NAME);
+	err |= fdt_property_string(fdt_out, "compatible",
+				   LUO_FDT_SESSION_COMPATIBLE);
+	err |= fdt_property(fdt_out, LUO_FDT_SESSION_HEADER, &header_ser_pa,
+			    sizeof(header_ser_pa));
+	err |= fdt_end_node(fdt_out);
+
+	if (err)
+		goto err_unpreserve;
+
+	luo_session_global.outgoing.header_ser = header_ser;
+	luo_session_global.outgoing.ser = (void *)(header_ser + 1);
+	luo_session_global.outgoing.active = true;
+
+	return 0;
+
+err_unpreserve:
+	kho_unpreserve_free(header_ser);
+	return err;
+}
+
+int __init luo_session_setup_incoming(void *fdt_in)
+{
+	struct luo_session_header_ser *header_ser;
+	int err, header_size, offset;
+	u64 header_ser_pa;
+	const void *ptr;
+
+	offset = fdt_subnode_offset(fdt_in, 0, LUO_FDT_SESSION_NODE_NAME);
+	if (offset < 0) {
+		pr_err("Unable to get session node: [%s]\n",
+		       LUO_FDT_SESSION_NODE_NAME);
+		return -EINVAL;
+	}
+
+	err = fdt_node_check_compatible(fdt_in, offset,
+					LUO_FDT_SESSION_COMPATIBLE);
+	if (err) {
+		pr_err("Session node incompatible [%s]\n",
+		       LUO_FDT_SESSION_COMPATIBLE);
+		return -EINVAL;
+	}
+
+	header_size = 0;
+	ptr = fdt_getprop(fdt_in, offset, LUO_FDT_SESSION_HEADER, &header_size);
+	if (!ptr || header_size != sizeof(u64)) {
+		pr_err("Unable to get session header '%s' [%d]\n",
+		       LUO_FDT_SESSION_HEADER, header_size);
+		return -EINVAL;
+	}
+
+	header_ser_pa = get_unaligned((u64 *)ptr);
+	header_ser = phys_to_virt(header_ser_pa);
+
+	luo_session_global.incoming.header_ser = header_ser;
+	luo_session_global.incoming.ser = (void *)(header_ser + 1);
+	luo_session_global.incoming.active = true;
+
+	return 0;
+}
+
+int luo_session_deserialize(void)
+{
+	struct luo_session_header *sh = &luo_session_global.incoming;
+	static bool is_deserialized;
+	static int err;
+
+	/* If has been deserialized, always return the same error code */
+	if (is_deserialized)
+		return err;
+
+	is_deserialized = true;
+	if (!sh->active)
+		return 0;
+
+	/*
+	 * Note on error handling:
+	 *
+	 * If deserialization fails (e.g., allocation failure or corrupt data),
+	 * we intentionally skip cleanup of sessions that were already restored.
+	 *
+	 * A partial failure leaves the preserved state inconsistent.
+	 * Implementing a safe "undo" to unwind complex dependencies (sessions,
+	 * files, hardware state) is error-prone and provides little value, as
+	 * the system is effectively in a broken state.
+	 *
+	 * We treat these resources as leaked. The expected recovery path is for
+	 * userspace to detect the failure and trigger a reboot, which will
+	 * reliably reset devices and reclaim memory.
+	 */
+	for (int i = 0; i < sh->header_ser->count; i++) {
+		struct luo_session *session;
+
+		session = luo_session_alloc(sh->ser[i].name);
+		if (IS_ERR(session)) {
+			pr_warn("Failed to allocate session [%s] during deserialization %pe\n",
+				sh->ser[i].name, session);
+			return PTR_ERR(session);
+		}
+
+		err = luo_session_insert(sh, session);
+		if (err) {
+			luo_session_free(session);
+			pr_warn("Failed to insert session [%s] %pe\n",
+				session->name, ERR_PTR(err));
+			return err;
+		}
+	}
+
+	kho_restore_free(sh->header_ser);
+	sh->header_ser = NULL;
+	sh->ser = NULL;
+
+	return 0;
+}
+
+int luo_session_serialize(void)
+{
+	struct luo_session_header *sh = &luo_session_global.outgoing;
+	struct luo_session *session;
+	int i = 0;
+
+	guard(rwsem_write)(&sh->rwsem);
+	list_for_each_entry(session, &sh->list, list) {
+		strscpy(sh->ser[i].name, session->name,
+			sizeof(sh->ser[i].name));
+		i++;
+	}
+	sh->header_ser->count = sh->count;
+
+	return 0;
+}
+
+/**
+ * luo_session_quiesce - Ensure no active sessions exist and lock session lists.
+ *
+ * Acquires exclusive write locks on both incoming and outgoing session lists.
+ * It then validates no sessions exist in either list.
+ *
+ * This mechanism is used during file handler un/registration to ensure that no
+ * sessions are currently using the handler, and no new sessions can be created
+ * while un/registration is in progress.
+ *
+ * Return:
+ * true  - System is quiescent (0 sessions) and locked.
+ * false - Active sessions exist. The locks are released internally.
+ */
+bool luo_session_quiesce(void)
+{
+	down_write(&luo_session_global.incoming.rwsem);
+	down_write(&luo_session_global.outgoing.rwsem);
+
+	if (luo_session_global.incoming.count ||
+	    luo_session_global.outgoing.count) {
+		up_write(&luo_session_global.outgoing.rwsem);
+		up_write(&luo_session_global.incoming.rwsem);
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * luo_session_resume - Unlock session lists and resume normal activity.
+ *
+ * Releases the exclusive locks acquired by a successful call to
+ * luo_session_quiesce().
+ */
+void luo_session_resume(void)
+{
+	up_write(&luo_session_global.outgoing.rwsem);
+	up_write(&luo_session_global.incoming.rwsem);
+}
-- 
2.52.0.rc2.455.g230fcf2819-goog

On Sat, Nov 22 2025, Pasha Tatashin wrote:

> Introduce concept of "Live Update Sessions" within the LUO framework.
> LUO sessions provide a mechanism to group and manage `struct file *`
> instances (representing file descriptors) that need to be preserved
> across a kexec-based live update.
>
> Each session is identified by a unique name and acts as a container
> for file objects whose state is critical to a userspace workload, such
> as a virtual machine or a high-performance database, aiming to maintain
> their functionality across a kernel transition.
>
> This groundwork establishes the framework for preserving file-backed
> state across kernel updates, with the actual file data preservation
> mechanisms to be implemented in subsequent patches.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>

With Mike's comments addressed,

Reviewed-by: Pratyush Yadav <pratyush@kernel.org>

[...]

-- 
Regards,
Pratyush Yadav

On Sat, Nov 22, 2025 at 05:23:31PM -0500, Pasha Tatashin wrote:
> Introduce concept of "Live Update Sessions" within the LUO framework.
> LUO sessions provide a mechanism to group and manage `struct file *`
> instances (representing file descriptors) that need to be preserved
> across a kexec-based live update.
> 
> Each session is identified by a unique name and acts as a container
> for file objects whose state is critical to a userspace workload, such
> as a virtual machine or a high-performance database, aiming to maintain
> their functionality across a kernel transition.
> 
> This groundwork establishes the framework for preserving file-backed
> state across kernel updates, with the actual file data preservation
> mechanisms to be implemented in subsequent patches.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/kho/abi/luo.h      |  71 +++++
>  include/uapi/linux/liveupdate.h  |   3 +
>  kernel/liveupdate/Makefile       |   3 +-
>  kernel/liveupdate/luo_core.c     |   9 +
>  kernel/liveupdate/luo_internal.h |  29 ++
>  kernel/liveupdate/luo_session.c  | 462 +++++++++++++++++++++++++++++++
>  6 files changed, 576 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/liveupdate/luo_session.c
> 

...

> +int __init luo_session_setup_outgoing(void *fdt_out)
> +{
> +	struct luo_session_header_ser *header_ser;
> +	void *outgoing_buffer;
> +	u64 header_ser_pa;
> +	int err;
> +
> +	outgoing_buffer = kho_alloc_preserve(LUO_SESSION_PGCNT << PAGE_SHIFT);
> +	if (IS_ERR(outgoing_buffer))
> +		return PTR_ERR(header_ser);

Should be 
		return PTR_ERR(outgoing_buffer);

Or, preferably, just drop outgoing_buffer and use header_ser everywhere.

> +	header_ser = outgoing_buffer;
> +	header_ser_pa = virt_to_phys(header_ser);
> +
> +	err = fdt_begin_node(fdt_out, LUO_FDT_SESSION_NODE_NAME);
> +	err |= fdt_property_string(fdt_out, "compatible",
> +				   LUO_FDT_SESSION_COMPATIBLE);
> +	err |= fdt_property(fdt_out, LUO_FDT_SESSION_HEADER, &header_ser_pa,
> +			    sizeof(header_ser_pa));
> +	err |= fdt_end_node(fdt_out);
> +
> +	if (err)
> +		goto err_unpreserve;
> +
> +	luo_session_global.outgoing.header_ser = header_ser;
> +	luo_session_global.outgoing.ser = (void *)(header_ser + 1);
> +	luo_session_global.outgoing.active = true;
> +
> +	return 0;
> +
> +err_unpreserve:
> +	kho_unpreserve_free(header_ser);
> +	return err;
> +}

...

> +int luo_session_deserialize(void)
> +{
> +	struct luo_session_header *sh = &luo_session_global.incoming;
> +	static bool is_deserialized;
> +	static int err;
> +
> +	/* If has been deserialized, always return the same error code */
> +	if (is_deserialized)
> +		return err;

is_deserialized and err are uninitialized here.

> +
> +	is_deserialized = true;
> +	if (!sh->active)
> +		return 0;
> +

...

> +/**
> + * luo_session_quiesce - Ensure no active sessions exist and lock session lists.
> + *
> + * Acquires exclusive write locks on both incoming and outgoing session lists.
> + * It then validates no sessions exist in either list.
> + *
> + * This mechanism is used during file handler un/registration to ensure that no
> + * sessions are currently using the handler, and no new sessions can be created
> + * while un/registration is in progress.

It makes sense to add something like this comment from luo_file.c here as well:

	 * This prevents registering new handlers while sessions are active or
	 * while deserialization is in progress.

> + *
> + * Return:
> + * true  - System is quiescent (0 sessions) and locked.
> + * false - Active sessions exist. The locks are released internally.
> + */

With those addressed:

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>


-- 
Sincerely yours,
Mike.

> > +     outgoing_buffer = kho_alloc_preserve(LUO_SESSION_PGCNT << PAGE_SHIFT);
> > +     if (IS_ERR(outgoing_buffer))
> > +             return PTR_ERR(header_ser);
>
> Should be
>                 return PTR_ERR(outgoing_buffer);

Thanks, fixed!

>
> Or, preferably, just drop outgoing_buffer and use header_ser everywhere.
>
> > +     header_ser = outgoing_buffer;
> > +     header_ser_pa = virt_to_phys(header_ser);
> > +
> > +     err = fdt_begin_node(fdt_out, LUO_FDT_SESSION_NODE_NAME);
> > +     err |= fdt_property_string(fdt_out, "compatible",
> > +                                LUO_FDT_SESSION_COMPATIBLE);
> > +     err |= fdt_property(fdt_out, LUO_FDT_SESSION_HEADER, &header_ser_pa,
> > +                         sizeof(header_ser_pa));
> > +     err |= fdt_end_node(fdt_out);
> > +
> > +     if (err)
> > +             goto err_unpreserve;
> > +
> > +     luo_session_global.outgoing.header_ser = header_ser;
> > +     luo_session_global.outgoing.ser = (void *)(header_ser + 1);
> > +     luo_session_global.outgoing.active = true;
> > +
> > +     return 0;
> > +
> > +err_unpreserve:
> > +     kho_unpreserve_free(header_ser);
> > +     return err;
> > +}
>
> ...
>
> > +int luo_session_deserialize(void)
> > +{
> > +     struct luo_session_header *sh = &luo_session_global.incoming;
> > +     static bool is_deserialized;
> > +     static int err;
> > +
> > +     /* If has been deserialized, always return the same error code */
> > +     if (is_deserialized)
> > +             return err;
>
> is_deserialized and err are uninitialized here.

These are global local variables. They are automatically initialized
to zero, and it is preferred in Linux source code to not set them to
zero.

> > +
> > +     is_deserialized = true;
> > +     if (!sh->active)
> > +             return 0;
> > +
>
> ...
>
> > +/**
> > + * luo_session_quiesce - Ensure no active sessions exist and lock session lists.
> > + *
> > + * Acquires exclusive write locks on both incoming and outgoing session lists.
> > + * It then validates no sessions exist in either list.
> > + *
> > + * This mechanism is used during file handler un/registration to ensure that no
> > + * sessions are currently using the handler, and no new sessions can be created
> > + * while un/registration is in progress.
>
> It makes sense to add something like this comment from luo_file.c here as well:
>
>          * This prevents registering new handlers while sessions are active or
>          * while deserialization is in progress.

Done

>
> > + *
> > + * Return:
> > + * true  - System is quiescent (0 sessions) and locked.
> > + * false - Active sessions exist. The locks are released internally.
> > + */
>
> With those addressed:
>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>
>
> --
> Sincerely yours,
> Mike.