On Wed, Jan 04, 2023 at 09:09:38AM -0700, Jens Axboe wrote: > If we split a bio marked with REQ_NOWAIT, then we can trigger spurious > EAGAIN if constituent parts of that split bio end up failing request > allocations. Parts will complete just fine, but just a single failure > in one of the chained bios will yield an EAGAIN final result for the > parent bio. > > Return EAGAIN early if we end up needing to split such a bio, which > allows for saner recovery handling. We're losing some performance here for large-ish single depth IO with nvme. We can get a little back by forcing to use the async worker earlier in the dispatch instead of getting all the way to the bio splitting, but the overhead to check for the condition (which is arbitrary decision anyway since we don't know the queue limits at io_uring prep time) mostly negates the gain. It's probably fine, though, since you can still hit peak b/w with just a little higher qdepth. This patch is a simple way to handle the problem, so looks good to me. Reviewed-by: Keith Busch <kbusch@xxxxxxxxxx>
