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@xxxxxxxxxx> > > > > 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
