https://bugzilla.kernel.org/show_bug.cgi?id=208827 --- Comment #22 from Jens Axboe (axboe@xxxxxxxxx) --- On 8/11/20 4:09 PM, Dave Chinner wrote: > On Tue, Aug 11, 2020 at 04:56:37PM -0400, Jeff Moyer wrote: >> Jens Axboe <axboe@xxxxxxxxx> writes: >> >>> So it seems to me like the file state is consistent, at least after the >>> run, and that this seems more likely to be a fio issue with short >>> read handling. >> >> Any idea why there was a short read, though? > > Yes. See: > > > https://lore.kernel.org/linux-xfs/20200807024211.GG2114@xxxxxxxxxxxxxxxxxxx/T/#maf3bd9325fb3ac0773089ca58609a2cea0386ddf > > It's a race between the readahead io completion marking pages > uptodate and unlocking them, and the io_uring worker function > getting woken on the first page being unlocked and running the > buffered read before the entire readahead IO completion has unlocked > all the pages in the IO. > > Basically, io_uring is re-running the IOCB_NOWAIT|IOCB_WAITQ IO when > there are still pages locked under IO. This will happen much more > frequently the larger the buffered read (these are only 64kB) and > the readahead windows are opened. > > Essentially, the io_uring buffered read needs to wait until _all_ > pages in the IO are marked up to date and unlocked, not just the > first one. And not just the last one, either - readahead can be > broken into multiple bios (because it spans extents) and there is no > guarantee of order of completion of the readahead bios given by the > readahead code.... Yes, it would ideally wait, or at least trigger on the last one. I'll see if I can improve that. For any of my testing, the amount of triggered short reads is minimal. For the verify case that we just ran, we're talking 8-12 ios out of 820 thousand, or 0.001% of them. So nothing that makes a performance difference in practical terms, though it would be nice to not hand back short reads if we can avoid it. Not for performance reasons, but for usage reasons. -- You are receiving this mail because: You are watching the assignee of the bug.
