Re: [PATCH -v4 1/4] mm/vmscan: fix anon-only reclaim evicting file pages when swappiness=max

[Michal Hocko]

On Fri 24-07-26 11:34:32, Ridong wrote:
> From: Ridong Chen <chenridong@xxxxxxxxxx>
> 
> As Qi mentioned [1], when swappiness=max (SWAPPINESS_ANON_ONLY) is set,
> the reclaim logic is expected to reclaim anonymous pages exclusively.
> However, due to the current ordering of checks in get_scan_count(),
> file pages may still be evicted if can_reclaim_anon_pages() returns
> false, which contradicts the semantics of SWAPPINESS_ANON_ONLY.
> 
> Reproducer in a cgroup holding 64M of file cache, with no swap configured:
> 
>   Before (file cache is wrongly evicted):
>     # cat memory.stat
>     anon 196608
>     file 67178496
>     pgscan_proactive 0
>     # echo "64M swappiness=max" > memory.reclaim
>     # cat memory.stat
>     anon 208896
>     file 4096                 <- page cache evicted
>     pgsteal_proactive 16400
>     pgscan_proactive 16400
> 
>   After (file cache is left intact):
>     # cat memory.stat
>     anon 200704
>     file 67178496
>     pgscan_proactive 0
>     # echo "64M swappiness=max" > memory.reclaim
>     -bash: echo: write error: Resource temporarily unavailable
>     # cat memory.stat
>     anon 208896
>     file 67178496             <- page cache untouched
>     pgsteal_proactive 0
>     pgscan_proactive 0
> 
> Fix this by bailing out early when SWAPPINESS_ANON_ONLY is set and no
> anonymous pages are reclaimable, before falling back to file reclaim.
> 
> [1] https://lore.kernel.org/cgroups/7ddf3eee-5fe2-45f7-8614-c8936a039e04@xxxxxxxxx/
> 
> Fixes: 68a1436bde00 ("mm: add swappiness=max arg to memory.reclaim for only anon reclaim")
> Suggested-by: Qi Zheng <qi.zheng@xxxxxxxxx>
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
> Reviewed-by: Qi Zheng <qi.zheng@xxxxxxxxx>
> Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 35c3bb15ae96..2c689682b952 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2501,6 +2501,23 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  	enum scan_balance scan_balance;
>  	enum lru_list lru;
>  
> +	/*
> +	 * Proactive reclaim initiated by userspace for anonymous memory only.
> +	 * SWAPPINESS_ANON_ONLY is set only on the proactive reclaim path, so
> +	 * warn if it shows up elsewhere. When anon cannot be reclaimed (e.g.
> +	 * no swap), bail out instead of falling back to evicting file pages,
> +	 * which would violate the anon-only semantics.
> +	 */
> +	if (swappiness == SWAPPINESS_ANON_ONLY) {
> +		WARN_ON_ONCE(!sc->proactive);
> +		if (!can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
> +			memset(nr, 0, sizeof(*nr) * NR_LRU_LISTS);
> +			return;
> +		}
> +		scan_balance = SCAN_ANON;
> +		goto out;

Rather than warning the code would be easier to understand
(SWAPPINESS_ANON_ONLY implying sc->proactive is very subtle assumption
that might change in the future) I would go with and explicit check.

Looking at how the code is structured currently doesn't the following
express the intention slightly better?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 35c3bb15ae96..4fd38e3f1b05 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2503,6 +2503,10 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 
 	/* If we have no swap space, do not bother scanning anon folios. */
 	if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
+		if (swappiness == SWAPPINESS_ANON_ONLY && sc->proactive) {
+			memset(nr, 0, sizeof(*nr) * NR_LRU_LISTS);
+			return;
+		}
 		scan_balance = SCAN_FILE;
 		goto out;
 	}
@@ -2521,7 +2525,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 
 	/* Proactive reclaim initiated by userspace for anonymous memory only */
 	if (swappiness == SWAPPINESS_ANON_ONLY) {
-		WARN_ON_ONCE(!sc->proactive);
 		scan_balance = SCAN_ANON;
 		goto out;
 	}
-- 
Michal Hocko
SUSE Labs