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

[Christoph Hellwig <hch@infradead.org>]

On Wed, May 28, 2025 at 03:22:26PM -0700, Darrick J. Wong wrote:
> 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.

Well, I'm all for using direct I/O loop devices by default.  But having
non-standard kernel hacks for that is pretty silly.  Can we just make
xfstests use direct I/O by default so that everyone uses the same
configuration?