Re: AIO read returns negative number for bytes read

[Avi Kivity <avi@scylladb.com>]

On 11/16/2015 09:27 PM, Jeff Moyer wrote:
> Hi Avi,
>
> Avi Kivity <avi@scylladb.com> writes:
>
>> Due to a bug in my program, I initiated a read beyond
>> eof. Specifically, the file size is 13002 bytes and the read offset is
>> 13312 (0x3400).
>>
>> I would expect such a read to return 0 bytes read, but io_getevents
>> returns -310, which is suspiciously equal to (13002 - 13312).
>>
>> I attach a reproducer.
>>
>> 4.2.5-201.fc22.x86_64
>>
>> Are my expectations incorrect, or is this a bug in aio or xfs?
> Your expectations are correct.  The bug was introduced by commit
> 9fe55eea7e4b4 (Fix race when checking i_size on direct i/o read).  I've
> CC'd the patch author and linux-fsdevel.  I'm not sure what the right
> fix is, given that the size checks were removed from the vfs to fix some
> race condition.  Unfortunately, the commit message doesn't really do a
> good job of explaining the race.  In order to save others time, here is
> a good explanation of the problem that commit is meant to fix, along
> with a reproducer:
>    http://marc.info/?l=linux-fsdevel&m=138641356614458&w=2
>
> Thanks for the great bug report, and sorry I have no solution to
> proffer.
>

Thanks.  I will await a fix with interest.