linux-kernel - Re: [PATCH] mm:fix duplicate accounting of free pages in should_reclaim

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Message-ID: <20250915132506.2889372-1-clm@meta.com>
Date: Mon, 15 Sep 2025 06:25:02 -0700
From: Chris Mason <clm@...a.com>
To: <liuqiqi@...inos.cn>
CC: Chris Mason <clm@...a.com>, <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm:fix duplicate accounting of free pages in should_reclaim_retry()

On Tue, 12 Aug 2025 15:02:10 +0800 liuqiqi@...inos.cn wrote:

> From: liuqiqi <liuqiqi@...inos.cn>
> 
> In the zone_reclaimable_pages() function, if the page counts for
> NR_ZONE_INACTIVE_FILE, NR_ZONE_ACTIVE_FILE, NR_ZONE_INACTIVE_ANON,
> and NR_ZONE_ACTIVE_ANON are all zero,
> the function returns the number of free pages as the result.
> 
> In this case, when should_reclaim_retry() calculates reclaimable pages,
> it will inadvertently double-count the free pages in its accounting.
> 
> static inline bool
> should_reclaim_retry(gfp_t gfp_mask, unsigned order,
>                      struct alloc_context *ac, int alloc_flags,
>                      bool did_some_progress, int *no_progress_loops)
> {
>         ...
>                 available = reclaimable = zone_reclaimable_pages(zone);
>                 available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
> 
> Signed-off-by: liuqiqi <liuqiqi@...inos.cn>
> ---
>  mm/vmscan.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 34410d24dc15..a9aaefdba7a2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -393,14 +393,7 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
>  	if (can_reclaim_anon_pages(NULL, zone_to_nid(zone), NULL))
>  		nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
>  			zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);
> -	/*
> -	 * If there are no reclaimable file-backed or anonymous pages,
> -	 * ensure zones with sufficient free pages are not skipped.
> -	 * This prevents zones like DMA32 from being ignored in reclaim
> -	 * scenarios where they can still help alleviate memory pressure.
> -	 */
> -	if (nr == 0)
> -		nr = zone_page_state_snapshot(zone, NR_FREE_PAGES);
> +
>  	return nr;
>  }
>  
> @@ -6417,7 +6410,7 @@ static bool allow_direct_reclaim(pg_data_t *pgdat)
>  		return true;
>  
>  	for_each_managed_zone_pgdat(zone, pgdat, i, ZONE_NORMAL) {
> -		if (!zone_reclaimable_pages(zone))
> +		if (!zone_reclaimable_pages(zone) && zone_page_state_snapshot(zone, NR_FREE_PAGES))
>  			continue;
>  

It looks like the logic got inverted when this code moved around.  Should this
be !zone_page_state_snapshot()?

-chris