linux-aio: explain why max batch is checked in laio_io_unplug()
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>
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@ -365,6 +365,12 @@ void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s,
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assert(s->io_q.plugged);
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s->io_q.plugged--;
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/*
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* Why max batch checking is performed here:
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* Another BDS may have queued requests with a higher dev_max_batch and
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* therefore in_queue could now exceed our dev_max_batch. Re-check the max
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* batch so we can honor our device's dev_max_batch.
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*/
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if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch) ||
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(!s->io_q.plugged &&
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!s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending))) {
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