Re: request_alignment vs file size, how to fix crash?

[Kevin Wolf]

Am 30.01.2020 um 12:30 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 30.01.2020 14:11, Kevin Wolf wrote:
> > Am 30.01.2020 um 11:40 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > 29.01.2020 21:01, Vladimir Sementsov-Ogievskiy wrote:
> > > > Hi!
> > > > 
> > > > I found a crash, which may be simply triggered for images unaligned to 
> > > > request_alignment:
> > > > 
> > > > # ./qemu-io --image-opts -c 'write 0 512' 
> > > > driver=blkdebug,align=4096,image.driver=null-co,image.size=512
> > > > qemu-io: block/io.c:1505: bdrv_aligned_pwritev: Assertion `end_sector 
> > > > <= bs->total_sectors || child->perm & BLK_PERM_RESIZE' failed.
> > > > Aborted (core dumped)
> > > > 
> > > > The problem is obvious: 512 is aligned to 4096 and becomes larger than 
> > > > file size.
> > > > 
> > > > I faced it after rebasing our downstream branches to newer Rhel 
> > > > versions. Seems that after some updates of alignment detection in 
> > > > file-posix.c, it started to detect 4096 alignment in our build 
> > > > environment, and iotest 152 started to crash (as it operates on file of 
> > > > 512 bytes).
> > > > 
> > > > My question is:
> > > > 
> > > > What is wrong? Should we restrict images to be aligned to 
> > > > request_alignment, or allow unaligned operations at EOF, if file is 
> > > > unaligned itself?
> > > > 
> > > 
> > > 
> > > The problem started with commit
> > > 
> > > commit a6b257a08e3d72219f03e461a52152672fec0612
> > > Author: Nir Soffer <address@hidden>
> > > Date:   Tue Aug 13 21:21:03 2019 +0300
> > > 
> > >      file-posix: Handle undetectable alignment
> > > 
> > > 
> > > It sets request_alignment to 4k, if probing of align=1 succeeded.. I 
> > > think it's wrong logic. It leads to crashes for images unaligned to 4k.
> > > 
> > > If we force alignment to be 4k, we at least should check that file size 
> > > is aligned to 4k. Otherwise our assumption is definitely wrong.
> > > 
> > > And still, I doubt that it's correct to force alignment to 4k, for 
> > > devices which doesn't request any alignment..
> > 
> > What backend is this? O_DIRECT with byte alignment sounds wrong, so I
> > wonder if your storage really can do this or whether we just failed to
> > detect the actual alignment.
> 
> The problem was disabled odirect in virtuozzo container which lead to byte 
> alignment. So, yes, it's on our part.

Oh, I see, so to QEMU it looked like it would do O_DIRECT and probing
was done, but what was actually opened was non-direct. Not sure if we
could possibly distinguish a situation like this from one where O_DIRECT
succeeds with byte alignment because the block was unallocated, but
would require larger alignment later.

> > I guess we could change the default to pick the largest size so that the
> > image size is still a multiple of it. But if the image size isn't even
> > aligned to 512 bytes, I think refusing to open the image with O_DIRECT
> > feels more correct (I would be okay with doing the same with > 512 byte
> > images, too, if the image size isn't a multiple of the alignment).
> > 
> 
> OK, I'll think about a patch for file-posix.c, and may be blkdebug too.
> 
> Also, we need to check it somewhere in generic layer too, to fail earlier 
> than assertion above.

Yes, I agree, it should be checked while opening the image.

Kevin