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Re: request_alignment vs file size, how to fix crash?
From: |
Kevin Wolf |
Subject: |
Re: request_alignment vs file size, how to fix crash? |
Date: |
Thu, 30 Jan 2020 12:11:44 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
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.
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).
Kevin