Re: [PATCH v5 2/6] mm: expose WB_DONTCACHE_DIRTY in bdi debugfs stats

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2026-10-19 at 11:07 -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.
> +	 */
> +	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),

Since this is for debugfs, I guess we don't have to worry about ABI.

Reviewed-by: Jeff Layton <jlayton@kernel.org>