Re: [PATCH 3/4] xfs/259: try to force loop device block size

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, May 22, 2025 at 10:19:00PM -0700, Christoph Hellwig wrote:
> On Wed, May 21, 2025 at 03:41:36PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > Starting with 6.15-rc1, loop devices created with directio mode enabled
> > will set their logical block size to whatever STATX_DIO_ALIGN on the
> > host filesystem reports.  If you happen to be running a kernel that
> > always sets up loop devices in directio mode
> 
> Such a kernel has some weird out of tree patches.  Why would we want
> to support that?

Welll... the only reason I patched the loop driver to turn ovn directio
by default is because writeback throttling for loop devices keeps
getting turned on and off randomly.  At this point I have NFI if
throttling is actually the desired behavior or not.  It makes fstests
crawl really slowly.

On one hand it seems bogus that a loopbacked filesystem with enough
dirty pages to trip the thresholds then gets throttled doing writeback
to the pagecache of the loop file, but OTOH it /is/ more dirty
pagecache.  Ultimately I think non-directio loop devices are stupid
especially when there are filesystems on top of them, but I bet there's
some user that would break if we suddenly started requiring directio
alignments.

Maybe RWF_DONTCACHE will solve this whenever it stabilizes.

--D