Re: AIO read returns negative number for bytes read

[Jeff Moyer <jmoyer@redhat.com>]

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.

Cheers,
Jeff