qemu-kvm/kvm-linux-aio-explain-why-max-batch-is-checked-in-laio_i.patch

From 99d33621440fd30e0da2974dafb0cd372334305a Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Thu, 9 Jun 2022 17:47:12 +0100
Subject: [PATCH 2/2] linux-aio: explain why max batch is checked in
 laio_io_unplug()

RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
RH-MergeRequest: 199: linux-aio: fix unbalanced plugged counter in laio_io_unplug()
RH-Commit: [2/2] 8617870ed70e3a57269f06eeb242d0fab79a66fb
RH-Bugzilla: 2105410
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
RH-Acked-by: Hanna Reitz <hreitz@redhat.com>
RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>

It may not be obvious why laio_io_unplug() checks max batch. I discussed
this with Stefano and have added a comment summarizing the reason.

Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20220609164712.1539045-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 99b969fbe105117f5af6060d3afef40ca39cc9c1)
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/linux-aio.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 77f17ad596..85650c4222 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -362,6 +362,12 @@ void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s,
     assert(s->io_q.plugged);
     s->io_q.plugged--;
 
+    /*
+     * Why max batch checking is performed here:
+     * Another BDS may have queued requests with a higher dev_max_batch and
+     * therefore in_queue could now exceed our dev_max_batch. Re-check the max
+     * batch so we can honor our device's dev_max_batch.
+     */
     if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch) ||
         (!s->io_q.plugged &&
          !s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending))) {
-- 
2.35.3