Re: Weird loop device behavior in 6.15-rc1?

[Christoph Hellwig <hch@infradead.org>]

On Mon, Apr 07, 2025 at 04:30:07PM -0700, Darrick J. Wong wrote:
> Hey Christoph,
> 
> I have a ... weird test setup where loop devices have directio enabled
> unconditionally on a system with 4k-lba disks, and now that I pulled
> down 6.15-rc1, I see failures in xfs/259:

Hmm, this works just fine for with a 4k LBA size NVMe setup on -rc1
with latest xfsprogs and xfstests for-next.

> Then trying to format an XFS filesystem fails:

That on the other hand I can reproduce locally.

> I think there's a bug in the loop driver where changing
> LO_FLAGS_DIRECT_IO doesn't actually try to change the O_DIRECT state of
> the underlying lo->lo_backing_file->f_flags.  So I can try to set a 2k
> block size on the loop dev, which turns off LO_FLAGS_DIRECT_IO but the
> fd is still open O_DIRECT so the writes fail.  But this isn't a
> regression in -rc1, so maybe this is the expected behavior?

This does look old, but also I would not call it expected.

> On 6.15-rc1, you actually /can/ change the sector size:

> But the backing file still has O_DIRECT on, so formatting fails:

Looks like the fact that fixing the silent failure to change the sector
size exposed the not clear O_DIRECT bug..

I'll cook up a patch to clear O_DIRECT.

> Thoughts?
> 
> --D
> 
> (/me notes that xfs/801 is failing across the board, and I don't know
> what changed about THPs in tmpfs but clearly something's corrupting
> memory.)

That one always failed for me because it uses a sysfs-dump tool that
simply doesn't seem to exist.