Re: [PATCH v2] xfs: enable WQ_MEM_RECLAIM on m_sync_workqueue

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2024-07-02 at 23:04 +1000, Dave Chinner wrote:
> On Tue, Jul 02, 2024 at 12:33:53PM +0000, Trond Myklebust wrote:
> > On Mon, 2024-07-01 at 10:13 -0400, Mike Snitzer wrote:
> > > On Mon, Jul 01, 2024 at 09:46:36AM +1000, Dave Chinner wrote:
> > > > IMO, the only sane way to ensure this sort of nested "back-end
> > > > page
> > > > cleaning submits front-end IO filesystem IO" mechanism works is
> > > > to
> > > > do something similar to the loop device. You most definitely
> > > > don't
> > > > want to be doing buffered IO (double caching is almost always
> > > > bad)
> > > > and you want to be doing async direct IO so that the submission
> > > > thread is not waiting on completion before the next IO is
> > > > submitted.
> > > 
> > > Yes, follow-on work is for me to revive the directio path for
> > > localio
> > > that ultimately wasn't pursued (or properly wired up) because it
> > > creates DIO alignment requirements on NFS client IO:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git/commit/?h=nfs-localio-for-6.11-testing&id=f6c9f51fca819a8af595a4eb94811c1f90051eab
> 
> I don't follow - this is page cache writeback. All the write IO from
> the bdi flusher thread should be page aligned, right? So why does DIO
> alignment matter here?
> 

There is no guarantee in NFS that writes from the flusher thread are
page aligned. If a page/folio is known to be up to date, we will
usually align writes to the boundaries, but we won't guarantee to do a
read-modify-write if that's not the case. Specifically, we will not do
so if the file is open for write-only.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com