On 3/30/22 9:53 AM, Jens Axboe wrote: > On 3/30/22 9:17 AM, Jens Axboe wrote: >> On 3/30/22 9:12 AM, Miklos Szeredi wrote: >>> On Wed, 30 Mar 2022 at 17:05, Jens Axboe <axboe@xxxxxxxxx> wrote: >>>> >>>> On 3/30/22 8:58 AM, Miklos Szeredi wrote: >>>>> Next issue: seems like file slot reuse is not working correctly. >>>>> Attached program compares reads using io_uring with plain reads of >>>>> proc files. >>>>> >>>>> In the below example it is using two slots alternately but the number >>>>> of slots does not seem to matter, read is apparently always using a >>>>> stale file (the prior one to the most recent open on that slot). See >>>>> how the sizes of the files lag by two lines: >>>>> >>>>> root@kvm:~# ./procreads >>>>> procreads: /proc/1/stat: ok (313) >>>>> procreads: /proc/2/stat: ok (149) >>>>> procreads: /proc/3/stat: read size mismatch 313/150 >>>>> procreads: /proc/4/stat: read size mismatch 149/154 >>>>> procreads: /proc/5/stat: read size mismatch 150/161 >>>>> procreads: /proc/6/stat: read size mismatch 154/171 >>>>> ... >>>>> >>>>> Any ideas? >>>> >>>> Didn't look at your code yet, but with the current tree, this is the >>>> behavior when a fixed file is used: >>>> >>>> At prep time, if the slot is valid it is used. If it isn't valid, >>>> assignment is deferred until the request is issued. >>>> >>>> Which granted is a bit weird. It means that if you do: >>>> >>>> <open fileA into slot 1, slot 1 currently unused><read slot 1> >>>> >>>> the read will read from fileA. But for: >>>> >>>> <open fileB into slot 1, slot 1 is fileA currently><read slot 1> >>>> >>>> since slot 1 is already valid at prep time for the read, the read will >>>> be from fileA again. >>>> >>>> Is this what you are seeing? It's definitely a bit confusing, and the >>>> only reason why I didn't change it is because it could potentially break >>>> applications. Don't think there's a high risk of that, however, so may >>>> indeed be worth it to just bite the bullet and the assignment is >>>> consistent (eg always done from the perspective of the previous >>>> dependent request having completed). >>>> >>>> Is this what you are seeing? >>> >>> Right, this explains it. Then the only workaround would be to wait >>> for the open to finish before submitting the read, but that would >>> defeat the whole point of using io_uring for this purpose. >> >> Honestly, I think we should just change it during this round, making it >> consistent with the "slot is unused" use case. The old use case is more >> more of a "it happened to work" vs the newer consistent behavior of "we >> always assign the file when execution starts on the request". >> >> Let me spin a patch, would be great if you could test. > > Something like this on top of the current tree should work. Can you > test? You can also just re-pull for-5.18/io_uring, it has been updated. A last minute edit make a 0 return from io_assign_file() which should've been 'true'... -- Jens Axboe
