Re: [PATCH] mm, vmstat: remove the NR_WRITEBACK_TEMP node_stat_item counter

[Vlastimil Babka <vbabka@suse.cz>]

On 6/25/25 17:51, Vlastimil Babka wrote:
> The only user of the counter (FUSE) was removed in commit 0c58a97f919c
> ("fuse: remove tmp folio for writebacks and internal rb tree") so follow
> the established pattern of removing the counter and hardcoding 0 in
> meminfo output, as done recently with NR_BOUNCE. Update documentation
> for procfs, including for the value for Bounce that was missed when
> removing its counter.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> The removal of the counter is straightforward. The reason for the large
> Cc list is that there is a comment in mm/page-writeback.c function
> wb_position_ratio() that mentions NR_WRITEBACK_TEMP, and just deleting
> the sentence feels to me it could be the wrong thing to do - maybe the
> strictlimit feature itself is now obsolete? It sure does mention FUSE
> as the main reason to exist, but commit 5a53748568f79 that introduced it
> also mentions slow USB sticks as a possibile scenario. Has that
> happened? I'm not familiar enough with this so I'd rather highlight this
> and ask for input here than make "git grep NR_WRITEBACK_TEMP" return
> nothing.

Thanks all for the input. Andrew, please squash in this fixup. The changelog
of that can be appended to the changelog of the original patch. Thanks.

----8<----
From 55d9070995010991abc0c6dbd68a8a53b5d622bc Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Mon, 7 Jul 2025 16:09:31 +0200
Subject: [PATCH] mm, vmstat: remove the NR_WRITEBACK_TEMP node_stat_item
 counter-fix

Also remove the mention of NR_WRITEBACK_TEMP implications from a comment
in wb_position_ratio(). The rest of the comment there about fuse setting
bdi->max_ratio to 1% is still correct.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/page-writeback.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 72b0ff0d4bae..3e248d1c3969 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1101,9 +1101,7 @@ static void wb_position_ratio(struct dirty_throttle_control *dtc)
 	 * such filesystems balance_dirty_pages always checks wb counters
 	 * against wb limits. Even if global "nr_dirty" is under "freerun".
 	 * This is especially important for fuse which sets bdi->max_ratio to
-	 * 1% by default. Without strictlimit feature, fuse writeback may
-	 * consume arbitrary amount of RAM because it is accounted in
-	 * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
+	 * 1% by default.
 	 *
 	 * Here, in wb_position_ratio(), we calculate pos_ratio based on
 	 * two values: wb_dirty and wb_thresh. Let's consider an example:
-- 
2.50.0