Re: [PATCH v7 1/5] mm: expose WB_DONTCACHE_DIRTY in bdi debugfs stats

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Oct 21, 2026 at 06:50:47AM -0400, Jeff Layton wrote:
> On Tue, 2026-10-20 at 18:03 -0400, Mike Snitzer wrote:
> > The per-bdi_writeback WB_DONTCACHE_DIRTY counter drives how much the
> > flusher writes back in response to an IOCB_DONTCACHE kick, but it is
> > not reported anywhere, so the DONTCACHE dirty backlog can only be
> > inferred from MemFree/Cached movement.
> > 
> > Report it as BdiDontcacheDirty in /sys/kernel/debug/bdi/<dev>/stats
> > and as WbDontcacheDirty in the per-wb stats, next to the
> > Reclaimable line.  Use wb_stat_sum() rather than wb_stat() to fold in
> > the per-cpu batches: the flusher targets the summed value and small
> > backlogs would otherwise read as zero.
> > 
> > Fixes: 88d6f128d06d ("mm: track DONTCACHE dirty pages per bdi_writeback")
> > Cc: Jeff Layton <jlayton@kernel.org>
> > Cc: Jens Axboe <axboe@kernel.dk>
> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: linux-mm@kvack.org
> > Assisted-by: LLM
> > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > ---
> >  mm/backing-dev.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index 18e999053bae..b8a2c27e07fa 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -46,6 +46,7 @@ struct wb_stats {
> >  	unsigned long nr_dirty_time;
> >  	unsigned long nr_writeback;
> >  	unsigned long nr_reclaimable;
> > +	unsigned long nr_dontcache_dirty;
> >  	unsigned long nr_dirtied;
> >  	unsigned long nr_written;
> >  	unsigned long dirty_thresh;
> > @@ -78,6 +79,12 @@ static void collect_wb_stats(struct wb_stats *stats,
> >  
> >  	stats->nr_writeback += wb_stat(wb, WB_WRITEBACK);
> >  	stats->nr_reclaimable += wb_stat(wb, WB_RECLAIMABLE);
> > +	/*
> > +	 * Sum the per-cpu batches: the dontcache flusher targets this
> > +	 * counter via wb_stat_sum() and small backlogs would otherwise
> > +	 * read as zero here.
> > +	 */
> 
> nit: I don't personally find this comment very helpful.

Meant to address "why not just use wb_stat()?". But will drop it (and
leave it to commit header to explain).

> > +	stats->nr_dontcache_dirty += wb_stat_sum(wb, WB_DONTCACHE_DIRTY);
> >  	stats->nr_dirtied += wb_stat(wb, WB_DIRTIED);
> >  	stats->nr_written += wb_stat(wb, WB_WRITTEN);
> >  	stats->wb_thresh += wb_calc_thresh(wb, stats->dirty_thresh);
> > @@ -125,6 +132,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
> >  	seq_printf(m,
> >  		   "BdiWriteback:       %10lu kB\n"
> >  		   "BdiReclaimable:     %10lu kB\n"
> > +		   "BdiDontcacheDirty:  %10lu kB\n"
> >  		   "BdiDirtyThresh:     %10lu kB\n"
> >  		   "DirtyThresh:        %10lu kB\n"
> >  		   "BackgroundThresh:   %10lu kB\n"
> > @@ -139,6 +147,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
> >  		   "state:              %10lx\n",
> >  		   K(stats.nr_writeback),
> >  		   K(stats.nr_reclaimable),
> > +		   K(stats.nr_dontcache_dirty),
> >  		   K(stats.wb_thresh),
> >  		   K(dirty_thresh),
> >  		   K(background_thresh),
> > @@ -163,6 +172,7 @@ static void wb_stats_show(struct seq_file *m, struct bdi_writeback *wb,
> >  		   "WbCgIno:           %10lu\n"
> >  		   "WbWriteback:       %10lu kB\n"
> >  		   "WbReclaimable:     %10lu kB\n"
> > +		   "WbDontcacheDirty:  %10lu kB\n"
> >  		   "WbDirtyThresh:     %10lu kB\n"
> >  		   "WbDirtied:         %10lu kB\n"
> >  		   "WbWritten:         %10lu kB\n"
> > @@ -179,6 +189,7 @@ static void wb_stats_show(struct seq_file *m, struct bdi_writeback *wb,
> >  #endif
> >  		   K(stats->nr_writeback),
> >  		   K(stats->nr_reclaimable),
> > +		   K(stats->nr_dontcache_dirty),
> >  		   K(stats->wb_thresh),
> >  		   K(stats->nr_dirtied),
> >  		   K(stats->nr_written),
> 
> Patch looks fine though:
> 
> Reviewed-by: Jeff Layton <jlayton@kernel.org>

Thanks!