From: Manish Mishra <manish.mishra@nutanix.com>
The kernel allocates extra metadata SKBs in case of a zerocopy send,
eventually used for zerocopy's notification mechanism. This metadata
memory is accounted for in the OPTMEM limit. The kernel queues
completion notifications on the socket error queue and this error queue
is freed when userspace reads it.
Usually, in the case of in-order processing, the kernel will batch the
notifications and merge the metadata into a single SKB and free the
rest. As a result, it never exceeds the OPTMEM limit. However, if there
is any out-of-order processing or intermittent zerocopy failures, this
error chain can grow significantly, exhausting the OPTMEM limit. As a
result, all new sendmsg requests fail to allocate any new SKB, leading
to an ENOBUF error. Depending on the amount of data queued before the
flush (i.e., large live migration iterations), even large OPTMEM limits
are prone to failure.
To work around this, if we encounter an ENOBUF error with a zerocopy
sendmsg, flush the error queue and retry once more.
Co-authored-by: Manish Mishra <manish.mishra@nutanix.com>
Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
---
include/io/channel-socket.h | 5 +++
io/channel-socket.c | 75 ++++++++++++++++++++++++++++++-------
2 files changed, 66 insertions(+), 14 deletions(-)
diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
index 26319fa98b..fcfd489c6c 100644
--- a/include/io/channel-socket.h
+++ b/include/io/channel-socket.h
@@ -50,6 +50,11 @@ struct QIOChannelSocket {
ssize_t zero_copy_queued;
ssize_t zero_copy_sent;
bool blocking;
+ /**
+ * This flag indicates whether any new data was successfully sent with
+ * zerocopy since the last qio_channel_socket_flush() call.
+ */
+ bool new_zero_copy_sent_success;
};
diff --git a/io/channel-socket.c b/io/channel-socket.c
index 8b30d5b7f7..7cd9f3666d 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -37,6 +37,12 @@
#define SOCKET_MAX_FDS 16
+#ifdef QEMU_MSG_ZEROCOPY
+static int qio_channel_socket_flush_internal(QIOChannel *ioc,
+ bool block,
+ Error **errp);
+#endif
+
SocketAddress *
qio_channel_socket_get_local_address(QIOChannelSocket *ioc,
Error **errp)
@@ -66,6 +72,7 @@ qio_channel_socket_new(void)
sioc->zero_copy_queued = 0;
sioc->zero_copy_sent = 0;
sioc->blocking = false;
+ sioc->new_zero_copy_sent_success = FALSE;
ioc = QIO_CHANNEL(sioc);
qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
@@ -618,6 +625,8 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
size_t fdsize = sizeof(int) * nfds;
struct cmsghdr *cmsg;
int sflags = 0;
+ bool blocking = sioc->blocking;
+ bool zerocopy_flushed_once = false;
memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
@@ -664,9 +673,24 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
goto retry;
case ENOBUFS:
if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
- error_setg_errno(errp, errno,
- "Process can't lock enough memory for using MSG_ZEROCOPY");
- return -1;
+ /**
+ * Socket error queueing may exhaust the OPTMEM limit. Try
+ * flushing the error queue once.
+ */
+ if (!zerocopy_flushed_once) {
+ ret = qio_channel_socket_flush_internal(ioc, blocking,
+ errp);
+ if (ret < 0) {
+ return -1;
+ }
+ zerocopy_flushed_once = TRUE;
+ goto retry;
+ } else {
+ error_setg_errno(errp, errno,
+ "Process can't lock enough memory for "
+ "using MSG_ZEROCOPY");
+ return -1;
+ }
}
break;
}
@@ -777,8 +801,9 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
#ifdef QEMU_MSG_ZEROCOPY
-static int qio_channel_socket_flush(QIOChannel *ioc,
- Error **errp)
+static int qio_channel_socket_flush_internal(QIOChannel *ioc,
+ bool block,
+ Error **errp)
{
QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
struct msghdr msg = {};
@@ -786,7 +811,6 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
struct cmsghdr *cm;
char control[CMSG_SPACE(sizeof(*serr))];
int received;
- int ret;
if (sioc->zero_copy_queued == sioc->zero_copy_sent) {
return 0;
@@ -796,16 +820,20 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
msg.msg_controllen = sizeof(control);
memset(control, 0, sizeof(control));
- ret = 1;
-
while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
if (received < 0) {
switch (errno) {
case EAGAIN:
- /* Nothing on errqueue, wait until something is available */
- qio_channel_wait(ioc, G_IO_ERR);
- continue;
+ if (block) {
+ /*
+ * Nothing on errqueue, wait until something is
+ * available.
+ */
+ qio_channel_wait(ioc, G_IO_ERR);
+ continue;
+ }
+ return 0;
case EINTR:
continue;
default:
@@ -843,13 +871,32 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
/* No errors, count successfully finished sendmsg()*/
sioc->zero_copy_sent += serr->ee_data - serr->ee_info + 1;
- /* If any sendmsg() succeeded using zero copy, return 0 at the end */
+ /* If any sendmsg() succeeded using zero copy, mark zerocopy success */
if (serr->ee_code != SO_EE_CODE_ZEROCOPY_COPIED) {
- ret = 0;
+ sioc->new_zero_copy_sent_success = TRUE;
}
}
- return ret;
+ return 0;
+}
+
+static int qio_channel_socket_flush(QIOChannel *ioc,
+ Error **errp)
+{
+ QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
+ int ret;
+
+ ret = qio_channel_socket_flush_internal(ioc, true, errp);
+ if (ret < 0) {
+ return ret;
+ }
+
+ if (sioc->new_zero_copy_sent_success) {
+ sioc->new_zero_copy_sent_success = FALSE;
+ return 0;
+ }
+
+ return 1;
}
#endif /* QEMU_MSG_ZEROCOPY */
--
2.43.7On Mon, Oct 13, 2025 at 09:21:22AM +0000, Tejus GK wrote:
> From: Manish Mishra <manish.mishra@nutanix.com>
>
> The kernel allocates extra metadata SKBs in case of a zerocopy send,
> eventually used for zerocopy's notification mechanism. This metadata
> memory is accounted for in the OPTMEM limit. The kernel queues
> completion notifications on the socket error queue and this error queue
> is freed when userspace reads it.
>
> Usually, in the case of in-order processing, the kernel will batch the
> notifications and merge the metadata into a single SKB and free the
> rest. As a result, it never exceeds the OPTMEM limit. However, if there
> is any out-of-order processing or intermittent zerocopy failures, this
> error chain can grow significantly, exhausting the OPTMEM limit. As a
> result, all new sendmsg requests fail to allocate any new SKB, leading
> to an ENOBUF error. Depending on the amount of data queued before the
> flush (i.e., large live migration iterations), even large OPTMEM limits
> are prone to failure.
>
> To work around this, if we encounter an ENOBUF error with a zerocopy
> sendmsg, flush the error queue and retry once more.
>
> Co-authored-by: Manish Mishra <manish.mishra@nutanix.com>
> Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
> ---
> include/io/channel-socket.h | 5 +++
> io/channel-socket.c | 75 ++++++++++++++++++++++++++++++-------
> 2 files changed, 66 insertions(+), 14 deletions(-)
>
> diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
> index 26319fa98b..fcfd489c6c 100644
> --- a/include/io/channel-socket.h
> +++ b/include/io/channel-socket.h
> @@ -50,6 +50,11 @@ struct QIOChannelSocket {
> ssize_t zero_copy_queued;
> ssize_t zero_copy_sent;
> bool blocking;
> + /**
> + * This flag indicates whether any new data was successfully sent with
> + * zerocopy since the last qio_channel_socket_flush() call.
> + */
> + bool new_zero_copy_sent_success;
> };
>
>
> diff --git a/io/channel-socket.c b/io/channel-socket.c
> index 8b30d5b7f7..7cd9f3666d 100644
> --- a/io/channel-socket.c
> +++ b/io/channel-socket.c
> @@ -37,6 +37,12 @@
>
> #define SOCKET_MAX_FDS 16
>
> +#ifdef QEMU_MSG_ZEROCOPY
> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
> + bool block,
> + Error **errp);
> +#endif
> +
> SocketAddress *
> qio_channel_socket_get_local_address(QIOChannelSocket *ioc,
> Error **errp)
> @@ -66,6 +72,7 @@ qio_channel_socket_new(void)
> sioc->zero_copy_queued = 0;
> sioc->zero_copy_sent = 0;
> sioc->blocking = false;
> + sioc->new_zero_copy_sent_success = FALSE;
>
> ioc = QIO_CHANNEL(sioc);
> qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
> @@ -618,6 +625,8 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
> size_t fdsize = sizeof(int) * nfds;
> struct cmsghdr *cmsg;
> int sflags = 0;
> + bool blocking = sioc->blocking;
> + bool zerocopy_flushed_once = false;
>
> memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
>
> @@ -664,9 +673,24 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
> goto retry;
> case ENOBUFS:
> if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> - error_setg_errno(errp, errno,
> - "Process can't lock enough memory for using MSG_ZEROCOPY");
> - return -1;
> + /**
> + * Socket error queueing may exhaust the OPTMEM limit. Try
> + * flushing the error queue once.
> + */
> + if (!zerocopy_flushed_once) {
> + ret = qio_channel_socket_flush_internal(ioc, blocking,
> + errp);
> + if (ret < 0) {
> + return -1;
> + }
> + zerocopy_flushed_once = TRUE;
> + goto retry;
> + } else {
> + error_setg_errno(errp, errno,
> + "Process can't lock enough memory for "
> + "using MSG_ZEROCOPY");
> + return -1;
> + }
> }
> break;
> }
> @@ -777,8 +801,9 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>
>
> #ifdef QEMU_MSG_ZEROCOPY
> -static int qio_channel_socket_flush(QIOChannel *ioc,
> - Error **errp)
> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
> + bool block,
> + Error **errp)
> {
> QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
> struct msghdr msg = {};
> @@ -786,7 +811,6 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
> struct cmsghdr *cm;
> char control[CMSG_SPACE(sizeof(*serr))];
> int received;
> - int ret;
>
> if (sioc->zero_copy_queued == sioc->zero_copy_sent) {
> return 0;
> @@ -796,16 +820,20 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
> msg.msg_controllen = sizeof(control);
> memset(control, 0, sizeof(control));
>
> - ret = 1;
> -
> while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
> received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
> if (received < 0) {
> switch (errno) {
> case EAGAIN:
> - /* Nothing on errqueue, wait until something is available */
> - qio_channel_wait(ioc, G_IO_ERR);
> - continue;
> + if (block) {
> + /*
> + * Nothing on errqueue, wait until something is
> + * available.
> + */
> + qio_channel_wait(ioc, G_IO_ERR);
> + continue;
Why G_IO_ERR ? If we're waiting for recvmsg() to become ready, then
it would need to be G_IO_IN we're waiting for.
> + }
> + return 0;
> case EINTR:
> continue;
> default:
With regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
> On 13 Oct 2025, at 3:08 PM, Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> !-------------------------------------------------------------------|
> CAUTION: External Email
>
> |-------------------------------------------------------------------!
>
> On Mon, Oct 13, 2025 at 09:21:22AM +0000, Tejus GK wrote:
>> From: Manish Mishra <manish.mishra@nutanix.com>
>>
>> The kernel allocates extra metadata SKBs in case of a zerocopy send,
>> eventually used for zerocopy's notification mechanism. This metadata
>> memory is accounted for in the OPTMEM limit. The kernel queues
>> completion notifications on the socket error queue and this error queue
>> is freed when userspace reads it.
>>
>> Usually, in the case of in-order processing, the kernel will batch the
>> notifications and merge the metadata into a single SKB and free the
>> rest. As a result, it never exceeds the OPTMEM limit. However, if there
>> is any out-of-order processing or intermittent zerocopy failures, this
>> error chain can grow significantly, exhausting the OPTMEM limit. As a
>> result, all new sendmsg requests fail to allocate any new SKB, leading
>> to an ENOBUF error. Depending on the amount of data queued before the
>> flush (i.e., large live migration iterations), even large OPTMEM limits
>> are prone to failure.
>>
>> To work around this, if we encounter an ENOBUF error with a zerocopy
>> sendmsg, flush the error queue and retry once more.
>>
>> Co-authored-by: Manish Mishra <manish.mishra@nutanix.com>
>> Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
>> ---
>> include/io/channel-socket.h | 5 +++
>> io/channel-socket.c | 75 ++++++++++++++++++++++++++++++-------
>> 2 files changed, 66 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
>> index 26319fa98b..fcfd489c6c 100644
>> --- a/include/io/channel-socket.h
>> +++ b/include/io/channel-socket.h
>> @@ -50,6 +50,11 @@ struct QIOChannelSocket {
>> ssize_t zero_copy_queued;
>> ssize_t zero_copy_sent;
>> bool blocking;
>> + /**
>> + * This flag indicates whether any new data was successfully sent with
>> + * zerocopy since the last qio_channel_socket_flush() call.
>> + */
>> + bool new_zero_copy_sent_success;
>> };
>>
>>
>> diff --git a/io/channel-socket.c b/io/channel-socket.c
>> index 8b30d5b7f7..7cd9f3666d 100644
>> --- a/io/channel-socket.c
>> +++ b/io/channel-socket.c
>> @@ -37,6 +37,12 @@
>>
>> #define SOCKET_MAX_FDS 16
>>
>> +#ifdef QEMU_MSG_ZEROCOPY
>> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
>> + bool block,
>> + Error **errp);
>> +#endif
>> +
>> SocketAddress *
>> qio_channel_socket_get_local_address(QIOChannelSocket *ioc,
>> Error **errp)
>> @@ -66,6 +72,7 @@ qio_channel_socket_new(void)
>> sioc->zero_copy_queued = 0;
>> sioc->zero_copy_sent = 0;
>> sioc->blocking = false;
>> + sioc->new_zero_copy_sent_success = FALSE;
>>
>> ioc = QIO_CHANNEL(sioc);
>> qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
>> @@ -618,6 +625,8 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>> size_t fdsize = sizeof(int) * nfds;
>> struct cmsghdr *cmsg;
>> int sflags = 0;
>> + bool blocking = sioc->blocking;
>> + bool zerocopy_flushed_once = false;
>>
>> memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
>>
>> @@ -664,9 +673,24 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>> goto retry;
>> case ENOBUFS:
>> if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
>> - error_setg_errno(errp, errno,
>> - "Process can't lock enough memory for using MSG_ZEROCOPY");
>> - return -1;
>> + /**
>> + * Socket error queueing may exhaust the OPTMEM limit. Try
>> + * flushing the error queue once.
>> + */
>> + if (!zerocopy_flushed_once) {
>> + ret = qio_channel_socket_flush_internal(ioc, blocking,
>> + errp);
>> + if (ret < 0) {
>> + return -1;
>> + }
>> + zerocopy_flushed_once = TRUE;
>> + goto retry;
>> + } else {
>> + error_setg_errno(errp, errno,
>> + "Process can't lock enough memory for "
>> + "using MSG_ZEROCOPY");
>> + return -1;
>> + }
>> }
>> break;
>> }
>> @@ -777,8 +801,9 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>>
>>
>> #ifdef QEMU_MSG_ZEROCOPY
>> -static int qio_channel_socket_flush(QIOChannel *ioc,
>> - Error **errp)
>> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
>> + bool block,
>> + Error **errp)
>> {
>> QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
>> struct msghdr msg = {};
>> @@ -786,7 +811,6 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
>> struct cmsghdr *cm;
>> char control[CMSG_SPACE(sizeof(*serr))];
>> int received;
>> - int ret;
>>
>> if (sioc->zero_copy_queued == sioc->zero_copy_sent) {
>> return 0;
>> @@ -796,16 +820,20 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
>> msg.msg_controllen = sizeof(control);
>> memset(control, 0, sizeof(control));
>>
>> - ret = 1;
>> -
>> while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
>> received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
>> if (received < 0) {
>> switch (errno) {
>> case EAGAIN:
>> - /* Nothing on errqueue, wait until something is available */
>> - qio_channel_wait(ioc, G_IO_ERR);
>> - continue;
>> + if (block) {
>> + /*
>> + * Nothing on errqueue, wait until something is
>> + * available.
>> + */
>> + qio_channel_wait(ioc, G_IO_ERR);
>> + continue;
>
> Why G_IO_ERR ? If we're waiting for recvmsg() to become ready, then
> it would need to be G_IO_IN we're waiting for.
Apologies for the delayed response. Please correct me if I am wrong,
https://docs.kernel.org/networking/msg_zerocopy.html#notification-reception
mentions, that in order to poll for notifications on the socket error queue, one need not set any flag in the events field, and the kernel in return would set POLLERR in the output, when there’s eventually a message in the notification queue.
From what I understand, the glib equivalent for POLLERR, is G_IO_ERR, which means we’d be waiting on the socket error queue until a notification comes up.
regards,
Tejus
On Tue, Oct 21, 2025 at 04:16:48PM +0000, Tejus GK wrote:
>
>
> > On 13 Oct 2025, at 3:08 PM, Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > !-------------------------------------------------------------------|
> > CAUTION: External Email
> >
> > |-------------------------------------------------------------------!
> >
> > On Mon, Oct 13, 2025 at 09:21:22AM +0000, Tejus GK wrote:
> >> From: Manish Mishra <manish.mishra@nutanix.com>
> >>
> >> The kernel allocates extra metadata SKBs in case of a zerocopy send,
> >> eventually used for zerocopy's notification mechanism. This metadata
> >> memory is accounted for in the OPTMEM limit. The kernel queues
> >> completion notifications on the socket error queue and this error queue
> >> is freed when userspace reads it.
> >>
> >> Usually, in the case of in-order processing, the kernel will batch the
> >> notifications and merge the metadata into a single SKB and free the
> >> rest. As a result, it never exceeds the OPTMEM limit. However, if there
> >> is any out-of-order processing or intermittent zerocopy failures, this
> >> error chain can grow significantly, exhausting the OPTMEM limit. As a
> >> result, all new sendmsg requests fail to allocate any new SKB, leading
> >> to an ENOBUF error. Depending on the amount of data queued before the
> >> flush (i.e., large live migration iterations), even large OPTMEM limits
> >> are prone to failure.
> >>
> >> To work around this, if we encounter an ENOBUF error with a zerocopy
> >> sendmsg, flush the error queue and retry once more.
> >>
> >> Co-authored-by: Manish Mishra <manish.mishra@nutanix.com>
> >> Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
> >> ---
> >> include/io/channel-socket.h | 5 +++
> >> io/channel-socket.c | 75 ++++++++++++++++++++++++++++++-------
> >> 2 files changed, 66 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
> >> index 26319fa98b..fcfd489c6c 100644
> >> --- a/include/io/channel-socket.h
> >> +++ b/include/io/channel-socket.h
> >> @@ -50,6 +50,11 @@ struct QIOChannelSocket {
> >> ssize_t zero_copy_queued;
> >> ssize_t zero_copy_sent;
> >> bool blocking;
> >> + /**
> >> + * This flag indicates whether any new data was successfully sent with
> >> + * zerocopy since the last qio_channel_socket_flush() call.
> >> + */
> >> + bool new_zero_copy_sent_success;
> >> };
> >>
> >>
> >> diff --git a/io/channel-socket.c b/io/channel-socket.c
> >> index 8b30d5b7f7..7cd9f3666d 100644
> >> --- a/io/channel-socket.c
> >> +++ b/io/channel-socket.c
> >> @@ -37,6 +37,12 @@
> >>
> >> #define SOCKET_MAX_FDS 16
> >>
> >> +#ifdef QEMU_MSG_ZEROCOPY
> >> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
> >> + bool block,
> >> + Error **errp);
> >> +#endif
> >> +
> >> SocketAddress *
> >> qio_channel_socket_get_local_address(QIOChannelSocket *ioc,
> >> Error **errp)
> >> @@ -66,6 +72,7 @@ qio_channel_socket_new(void)
> >> sioc->zero_copy_queued = 0;
> >> sioc->zero_copy_sent = 0;
> >> sioc->blocking = false;
> >> + sioc->new_zero_copy_sent_success = FALSE;
> >>
> >> ioc = QIO_CHANNEL(sioc);
> >> qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
> >> @@ -618,6 +625,8 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
> >> size_t fdsize = sizeof(int) * nfds;
> >> struct cmsghdr *cmsg;
> >> int sflags = 0;
> >> + bool blocking = sioc->blocking;
> >> + bool zerocopy_flushed_once = false;
> >>
> >> memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
> >>
> >> @@ -664,9 +673,24 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
> >> goto retry;
> >> case ENOBUFS:
> >> if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> >> - error_setg_errno(errp, errno,
> >> - "Process can't lock enough memory for using MSG_ZEROCOPY");
> >> - return -1;
> >> + /**
> >> + * Socket error queueing may exhaust the OPTMEM limit. Try
> >> + * flushing the error queue once.
> >> + */
> >> + if (!zerocopy_flushed_once) {
> >> + ret = qio_channel_socket_flush_internal(ioc, blocking,
> >> + errp);
> >> + if (ret < 0) {
> >> + return -1;
> >> + }
> >> + zerocopy_flushed_once = TRUE;
> >> + goto retry;
> >> + } else {
> >> + error_setg_errno(errp, errno,
> >> + "Process can't lock enough memory for "
> >> + "using MSG_ZEROCOPY");
> >> + return -1;
> >> + }
> >> }
> >> break;
> >> }
> >> @@ -777,8 +801,9 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
> >>
> >>
> >> #ifdef QEMU_MSG_ZEROCOPY
> >> -static int qio_channel_socket_flush(QIOChannel *ioc,
> >> - Error **errp)
> >> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
> >> + bool block,
> >> + Error **errp)
> >> {
> >> QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
> >> struct msghdr msg = {};
> >> @@ -786,7 +811,6 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
> >> struct cmsghdr *cm;
> >> char control[CMSG_SPACE(sizeof(*serr))];
> >> int received;
> >> - int ret;
> >>
> >> if (sioc->zero_copy_queued == sioc->zero_copy_sent) {
> >> return 0;
> >> @@ -796,16 +820,20 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
> >> msg.msg_controllen = sizeof(control);
> >> memset(control, 0, sizeof(control));
> >>
> >> - ret = 1;
> >> -
> >> while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
> >> received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
> >> if (received < 0) {
> >> switch (errno) {
> >> case EAGAIN:
> >> - /* Nothing on errqueue, wait until something is available */
> >> - qio_channel_wait(ioc, G_IO_ERR);
> >> - continue;
> >> + if (block) {
> >> + /*
> >> + * Nothing on errqueue, wait until something is
> >> + * available.
> >> + */
> >> + qio_channel_wait(ioc, G_IO_ERR);
> >> + continue;
> >
> > Why G_IO_ERR ? If we're waiting for recvmsg() to become ready, then
> > it would need to be G_IO_IN we're waiting for.
>
> Apologies for the delayed response. Please correct me if I am wrong,
> https://docs.kernel.org/networking/msg_zerocopy.html#notification-reception
> mentions, that in order to poll for notifications on the socket error queue, one need not set any flag in the events field, and the kernel in return would set POLLERR in the output, when there’s eventually a message in the notification queue.
> From what I understand, the glib equivalent for POLLERR, is G_IO_ERR, which means we’d be waiting on the socket error queue until a notification comes up.
Ah I see. That's rather non-obvious. Can you put a comment in the code
to the effect that use of MSG_ERRQUEUE requires to you poll on G_IO_ERR
instead of the normal G_IO_IN.
With regards,
Daniel
--
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> On 24 Oct 2025, at 5:49 PM, Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> !-------------------------------------------------------------------|
> CAUTION: External Email
>
> |-------------------------------------------------------------------!
>
> On Tue, Oct 21, 2025 at 04:16:48PM +0000, Tejus GK wrote:
>>
>>
>>> On 13 Oct 2025, at 3:08 PM, Daniel P. Berrangé <berrange@redhat.com> wrote:
>>>
>>> !-------------------------------------------------------------------|
>>> CAUTION: External Email
>>>
>>> |-------------------------------------------------------------------!
>>>
>>> On Mon, Oct 13, 2025 at 09:21:22AM +0000, Tejus GK wrote:
>>>> From: Manish Mishra <manish.mishra@nutanix.com>
>>>>
>>>> The kernel allocates extra metadata SKBs in case of a zerocopy send,
>>>> eventually used for zerocopy's notification mechanism. This metadata
>>>> memory is accounted for in the OPTMEM limit. The kernel queues
>>>> completion notifications on the socket error queue and this error queue
>>>> is freed when userspace reads it.
>>>>
>>>> Usually, in the case of in-order processing, the kernel will batch the
>>>> notifications and merge the metadata into a single SKB and free the
>>>> rest. As a result, it never exceeds the OPTMEM limit. However, if there
>>>> is any out-of-order processing or intermittent zerocopy failures, this
>>>> error chain can grow significantly, exhausting the OPTMEM limit. As a
>>>> result, all new sendmsg requests fail to allocate any new SKB, leading
>>>> to an ENOBUF error. Depending on the amount of data queued before the
>>>> flush (i.e., large live migration iterations), even large OPTMEM limits
>>>> are prone to failure.
>>>>
>>>> To work around this, if we encounter an ENOBUF error with a zerocopy
>>>> sendmsg, flush the error queue and retry once more.
>>>>
>>>> Co-authored-by: Manish Mishra <manish.mishra@nutanix.com>
>>>> Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
>>>> ---
>>>> include/io/channel-socket.h | 5 +++
>>>> io/channel-socket.c | 75 ++++++++++++++++++++++++++++++-------
>>>> 2 files changed, 66 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
>>>> index 26319fa98b..fcfd489c6c 100644
>>>> --- a/include/io/channel-socket.h
>>>> +++ b/include/io/channel-socket.h
>>>> @@ -50,6 +50,11 @@ struct QIOChannelSocket {
>>>> ssize_t zero_copy_queued;
>>>> ssize_t zero_copy_sent;
>>>> bool blocking;
>>>> + /**
>>>> + * This flag indicates whether any new data was successfully sent with
>>>> + * zerocopy since the last qio_channel_socket_flush() call.
>>>> + */
>>>> + bool new_zero_copy_sent_success;
>>>> };
>>>>
>>>>
>>>> diff --git a/io/channel-socket.c b/io/channel-socket.c
>>>> index 8b30d5b7f7..7cd9f3666d 100644
>>>> --- a/io/channel-socket.c
>>>> +++ b/io/channel-socket.c
>>>> @@ -37,6 +37,12 @@
>>>>
>>>> #define SOCKET_MAX_FDS 16
>>>>
>>>> +#ifdef QEMU_MSG_ZEROCOPY
>>>> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
>>>> + bool block,
>>>> + Error **errp);
>>>> +#endif
>>>> +
>>>> SocketAddress *
>>>> qio_channel_socket_get_local_address(QIOChannelSocket *ioc,
>>>> Error **errp)
>>>> @@ -66,6 +72,7 @@ qio_channel_socket_new(void)
>>>> sioc->zero_copy_queued = 0;
>>>> sioc->zero_copy_sent = 0;
>>>> sioc->blocking = false;
>>>> + sioc->new_zero_copy_sent_success = FALSE;
>>>>
>>>> ioc = QIO_CHANNEL(sioc);
>>>> qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
>>>> @@ -618,6 +625,8 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>>>> size_t fdsize = sizeof(int) * nfds;
>>>> struct cmsghdr *cmsg;
>>>> int sflags = 0;
>>>> + bool blocking = sioc->blocking;
>>>> + bool zerocopy_flushed_once = false;
>>>>
>>>> memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
>>>>
>>>> @@ -664,9 +673,24 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>>>> goto retry;
>>>> case ENOBUFS:
>>>> if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
>>>> - error_setg_errno(errp, errno,
>>>> - "Process can't lock enough memory for using MSG_ZEROCOPY");
>>>> - return -1;
>>>> + /**
>>>> + * Socket error queueing may exhaust the OPTMEM limit. Try
>>>> + * flushing the error queue once.
>>>> + */
>>>> + if (!zerocopy_flushed_once) {
>>>> + ret = qio_channel_socket_flush_internal(ioc, blocking,
>>>> + errp);
>>>> + if (ret < 0) {
>>>> + return -1;
>>>> + }
>>>> + zerocopy_flushed_once = TRUE;
>>>> + goto retry;
>>>> + } else {
>>>> + error_setg_errno(errp, errno,
>>>> + "Process can't lock enough memory for "
>>>> + "using MSG_ZEROCOPY");
>>>> + return -1;
>>>> + }
>>>> }
>>>> break;
>>>> }
>>>> @@ -777,8 +801,9 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>>>>
>>>>
>>>> #ifdef QEMU_MSG_ZEROCOPY
>>>> -static int qio_channel_socket_flush(QIOChannel *ioc,
>>>> - Error **errp)
>>>> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
>>>> + bool block,
>>>> + Error **errp)
>>>> {
>>>> QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
>>>> struct msghdr msg = {};
>>>> @@ -786,7 +811,6 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
>>>> struct cmsghdr *cm;
>>>> char control[CMSG_SPACE(sizeof(*serr))];
>>>> int received;
>>>> - int ret;
>>>>
>>>> if (sioc->zero_copy_queued == sioc->zero_copy_sent) {
>>>> return 0;
>>>> @@ -796,16 +820,20 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
>>>> msg.msg_controllen = sizeof(control);
>>>> memset(control, 0, sizeof(control));
>>>>
>>>> - ret = 1;
>>>> -
>>>> while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
>>>> received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
>>>> if (received < 0) {
>>>> switch (errno) {
>>>> case EAGAIN:
>>>> - /* Nothing on errqueue, wait until something is available */
>>>> - qio_channel_wait(ioc, G_IO_ERR);
>>>> - continue;
>>>> + if (block) {
>>>> + /*
>>>> + * Nothing on errqueue, wait until something is
>>>> + * available.
>>>> + */
>>>> + qio_channel_wait(ioc, G_IO_ERR);
>>>> + continue;
>>>
>>> Why G_IO_ERR ? If we're waiting for recvmsg() to become ready, then
>>> it would need to be G_IO_IN we're waiting for.
>>
>> Apologies for the delayed response. Please correct me if I am wrong,
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.kernel.org_networking_msg-5Fzerocopy.html-23notification-2Dreception&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=v-j9kkKctOUcEWmsO1q0U5Gh8gvY15hrX0OGS1tEpJ0&m=iaYkvpxmwR4NUsJEXl9wTk6PrRBhMy0DqfuRqsLaPgvu9SlaZ0aK_Bg_AchDD34r&s=t020DODDta7ptJ_UWPqIShTbeXqzeN3NcIIRnk7_dD8&e=
>> mentions, that in order to poll for notifications on the socket error queue, one need not set any flag in the events field, and the kernel in return would set POLLERR in the output, when there’s eventually a message in the notification queue.
>> From what I understand, the glib equivalent for POLLERR, is G_IO_ERR, which means we’d be waiting on the socket error queue until a notification comes up.
>
> Ah I see. That's rather non-obvious. Can you put a comment in the code
> to the effect that use of MSG_ERRQUEUE requires to you poll on G_IO_ERR
> instead of the normal G_IO_IN.
Ack, I’ll add a comment in the next patch-series. Thanks!
regards,
tejus
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