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

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Message-Id: <20250812070210.1624218-1-liuqiqi@kylinos.cn>
Date: Tue, 12 Aug 2025 15:02:10 +0800
From: liuqiqi@...inos.cn
To: akpm@...ux-foundation.org
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	liuqiqi <liuqiqi@...inos.cn>
Subject: [PATCH] mm:fix duplicate accounting of free pages in should_reclaim_retry()

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;
 
 		pfmemalloc_reserve += min_wmark_pages(zone);
-- 
2.25.1