io: flush zerocopy socket error queue on sendmsg failure due to ENOBUF

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>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
[DB: change TRUE/FALSE to true/false for 'bool' type;
     add more #ifdef QEMU_MSG_ZEROCOPY blocks]
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

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;
 };