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

[Stefano Garzarella]

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 
Cc: Kevin Wolf 
Signed-off-by: Stefan Hajnoczi 
---
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,
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(>io_q.pending))) {
--
2.36.1



I should have added that...

Reviewed-by: Stefano Garzarella 

[PATCH 2/2] linux-aio: explain why max batch is checked in laio_io_unplug()

[Stefan Hajnoczi]

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 
Cc: Kevin Wolf 
Signed-off-by: Stefan Hajnoczi 
---
 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,
 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(>io_q.pending))) {
-- 
2.36.1