Re: [PATCH] writeback: Account the number of pages written back

[Matthew Wilcox]

On Tue, Jul 04, 2023 at 07:37:17AM +1000, Dave Chinner wrote:
> On Mon, Jul 03, 2023 at 03:13:15AM +0100, Matthew Wilcox wrote:
> > On Sun, Jul 02, 2023 at 01:06:15PM -0700, Andrew Morton wrote:
> > > On Wed, 28 Jun 2023 19:55:48 +0100 "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> wrote:
> > > 
> > > > nr_to_write is a count of pages, so we need to decrease it by the number
> > > > of pages in the folio we just wrote, not by 1.  Most callers specify
> > > > either LONG_MAX or 1, so are unaffected, but writeback_sb_inodes()
> > > > might end up writing 512x as many pages as it asked for.
> > > 
> > > 512 is a big number,  Should we backport this?
> > 
> > I'm really not sure.  Maybe?  I'm hoping one of the bots comes up with a
> > meaningful performance change as a result of this patch and we find out.
> 
> XFS is the only filesystem this would affect, right? AFAIA, nothing
> else enables large folios and uses writeback through
> write_cache_pages() at this point...

Good point.  Still, Intel's 0day has squawked about a loss of performance
when large folios have _stopped_ being used, so they are at least testing
with XFS.