Re: [PATCH] mm/vmscan: reclaim every zone in per-node proactive reclaim

[Kunwu Chan]

On Mon, 19 Oct 2026 14:50:02 -0400 "Gregory Price (Meta)" <gourry@xxxxxxxxxx> wrote:

> user_proactive_reclaim() limits per-node reclaim to
> gfp_zone(GFP_KERNEL), which is ZONE_NORMAL. Proactive reclaim serves no
> allocation, but that limit makes shrink_node() skip every ZONE_MOVABLE
> folio, so writing to /sys/devices/system/node/nodeN/reclaim never
> reclaims a node's movable memory. A movable-only node, such as dax/kmem
> or hotplugged memory onlined movable, reclaims nothing and fails with
> -EAGAIN.
> 
> To reproduce, fill a movable-only node with clean page cache and write
> 200M to its nodeN/reclaim: the write fails with -EAGAIN and the page
> cache stays. The same write to a ZONE_NORMAL node frees 200MB.
> 
> Scan every zone, as memory.reclaim does.
> 
> Fixes: b980077899ea4 ("mm: introduce per-node proactive reclaim interface")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e82ba737c028d..90f7a7bf4e426 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -8180,7 +8180,7 @@ int user_proactive_reclaim(char *buf,
>  		} else {
>  			struct scan_control sc = {
>  				.gfp_mask = current_gfp_context(gfp_mask),
> -				.reclaim_idx = gfp_zone(gfp_mask),
> +				.reclaim_idx = MAX_NR_ZONES - 1,
>  				.proactive_swappiness = swappiness == -1 ? NULL : &swappiness,
>  				.priority = DEF_PRIORITY,
>  				.may_writepage = 1,
> -- 
> 2.53.0-Meta
> 
> 

reclaim_idx = MAX_NR_ZONES - 1 is the right fix — proactive reclaim
isn't serving an allocation, so gfp_zone(GFP_KERNEL) needlessly caps
the scan.  GFP_HIGHUSER_MOVABLE would also work for the node path
via gfp_zone(), but that's indirect.

Reviewed-by: Kunwu Chan <chentao@xxxxxxxxxx>