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Re: [PATCH 2/2] linux-aio: explain why max batch is checked in laio_io_u

From: Stefano Garzarella
Subject: Re: [PATCH 2/2] linux-aio: explain why max batch is checked in laio_io_unplug()
Date: Thu, 9 Jun 2022 19:47:07 +0200

On Thu, Jun 09, 2022 at 05:47:12PM +0100, Stefan Hajnoczi wrote:
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>
block/linux-aio.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 6078da7e42..9c2393a2f7 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -365,6 +365,12 @@ void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s,

+    /*
+     * 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))) {

I should have added that...

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

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