[Qemu-devel] [PATCH 2/4] linux-aio: handling -EAGAIN for !s->io_q.plugged case

[Ming Lei <ming.lei@canonical.com>]

Previously -EAGAIN is simply ignored for !s->io_q.plugged case,
and sometimes it is easy to cause -EIO to VM, such as NVME device.

This patch handles -EAGAIN by io queue for !s->io_q.plugged case,
and it will be retried in following aio completion cb.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
 block/linux-aio.c |   22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 4cdf507..0e21f76 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -238,6 +238,11 @@ static int ioq_enqueue(struct qemu_laio_state *s, struct iocb *iocb)
     s->io_q.iocbs[idx++] = iocb;
     s->io_q.idx = idx;
 
+    /* don't submit until next completion for -EAGAIN of non plug case */
+    if (unlikely(!s->io_q.plugged)) {
+        return 0;
+    }
+
     /* submit immediately if queue depth is above 2/3 */
     if (idx > s->io_q.size * 2 / 3) {
         return ioq_submit(s, true);
@@ -305,10 +310,25 @@ BlockDriverAIOCB *laio_submit(BlockDriverState *bs, void *aio_ctx, int fd,
     io_set_eventfd(&laiocb->iocb, event_notifier_get_fd(&s->e));
 
     if (!s->io_q.plugged) {
-        if (io_submit(s->ctx, 1, &iocbs) < 0) {
+        int ret;
+
+        if (!s->io_q.idx) {
+            ret = io_submit(s->ctx, 1, &iocbs);
+        } else {
+            ret = -EAGAIN;
+        }
+        /*
+         * Switch to queue mode until -EAGAIN is handled, we suppose
+         * there is always uncompleted I/O, so try to enqueue it first,
+         * and will be submitted again in following aio completion cb.
+         */
+        if (ret == -EAGAIN) {
+            goto enqueue;
+        } else if (ret < 0) {
             goto out_free_aiocb;
         }
     } else {
+ enqueue:
         if (ioq_enqueue(s, iocbs) < 0) {
             goto out_free_aiocb;
         }
-- 
1.7.9.5