Re: [PATCH v8 00/16] mm: mglru: fix swappiness behavior

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 14 Nov 2026 02:55:25 +0800 "Barry Song (Xiaomi)" <baohua@kernel.org> wrote:

> Classic LRU respects swappiness, while MGLRU only weakly
> honors it when selecting the scan type. The fallback logic
> can easily override the bias established by
> get_type_to_scan(). As a result, swappiness has a much
> smaller overall impact when changing from 1 to 200, as
> shown by the data presented later in this cover letter.
> 
> In this patchset, patches 1-8 from Kairui introduce a model that uses
> refaults to determine nr_to_scan[] for both file and anon in MGLRU.
> For each reclaim operation, we calculate how many file and anon
> folios to scan, similar to CLRU, rather than relying on
> get_type_to_scan() and fallback mechanisms to decide what to
> reclaim, which have proven rather unreliable in honoring swappiness.
> This model also allows scanning and reclaiming from the two youngest
> generations, including the youngest generation itself. This can help
> resolve OOM situations[1] and reduce the need for aging.
> 
> ...
>

Thanks, I've updated mm.git's mm-new branch to this version.  I expect
I'll advance the series into mm-unstable (hence linux-next) over the
weekend.

> v8:
> * Fixed Gaofei Lyu's name;
> * for debugfs proactive reclaim, use DEF_PRIORITY;
> * refine no-populatable aging (limit frequency, allow an aging
>   after eviction returns 0 scanned)
> * refine "restrict eviction accounting to eligible zones" for
>   potential premature OOM;
> * Collect Baolin's reviewed-by tag, thanks!

Here's how v8 altered mm.git:


 mm/vmscan.c |   34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

--- a/mm/vmscan.c~b
+++ a/mm/vmscan.c
@@ -4034,8 +4034,6 @@ static void try_to_inc_min_seq(struct lr
 
 	/* see the comment on lru_gen_folio */
 	if (swappiness && swappiness <= MAX_SWAPPINESS) {
-		unsigned long seq = lrugen->max_seq - MIN_NR_GENS;
-
 		if (min_seq[LRU_GEN_ANON] > seq && min_seq[LRU_GEN_FILE] < seq)
 			min_seq[LRU_GEN_ANON] = seq;
 		else if (min_seq[LRU_GEN_FILE] > seq && min_seq[LRU_GEN_ANON] < seq)
@@ -4246,7 +4244,13 @@ static bool lruvec_is_reclaimable(struct
 	if (mem_cgroup_below_min(NULL, memcg))
 		return false;
 
-	if (!lruvec_is_sizable(lruvec, sc))
+	/*
+	 * As in __alloc_pages_may_oom(), avoid triggering OOM for low-zone
+	 * allocations. Also, defer the LRU size check until reclaim reaches
+	 * a very low priority to avoid premature OOM.
+	 */
+	if (sc->priority <= 1 && sc->reclaim_idx >= ZONE_NORMAL &&
+	    !lruvec_is_sizable(lruvec, sc))
 		return false;
 
 	gen = lru_gen_from_seq(evictable_min_seq(min_seq, swappiness));
@@ -5220,6 +5224,13 @@ static bool try_to_shrink_lruvec(struct
 	bool need_rotate = false, should_age = false;
 	int swappiness = get_swappiness(lruvec, sc);
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
+	/*
+	 * Avoid aging the lagging type under extreme swappiness,
+	 * as reclaim of the unpreferred type is disabled until
+	 * DEF_PRIORITY / 2.
+	 */
+	bool allow_no_populatable_age = (sc->priority <= DEF_PRIORITY / 2) ||
+		(swappiness > MIN_SWAPPINESS + 1 && swappiness < MAX_SWAPPINESS);
 
 	/*
 	 * Proactive reclaim initiated by userspace for anonymous memory only.
@@ -5238,8 +5249,9 @@ static bool try_to_shrink_lruvec(struct
 
 	while (nr[LRU_GEN_ANON] > 0 || nr[LRU_GEN_FILE] > 0) {
 		unsigned long orig_file, orig_anon;
-		DEFINE_MAX_SEQ(lruvec);
+		unsigned long max_seq;
 		bool no_populatable;
+		long scanned;
 
 		if (mem_cgroup_below_min(sc->target_mem_cgroup, memcg)) {
 			need_rotate = true;
@@ -5248,8 +5260,7 @@ static bool try_to_shrink_lruvec(struct
 
 		orig_anon = nr[LRU_GEN_ANON];
 		orig_file = nr[LRU_GEN_FILE];
-		if (!evict_folio_lists(nr, lruvec, sc, swappiness, MIN_LRU_BATCH))
-			break;
+		scanned = evict_folio_lists(nr, lruvec, sc, swappiness, MIN_LRU_BATCH);
 
 		if (should_abort_scan(lruvec, sc))
 			break;
@@ -5261,14 +5272,20 @@ static bool try_to_shrink_lruvec(struct
 		if (root_reclaim(sc) && should_age)
 			break;
 
-		no_populatable = (nr[LRU_GEN_ANON] && nr[LRU_GEN_ANON] == orig_anon) ||
-				 (nr[LRU_GEN_FILE] && nr[LRU_GEN_FILE] == orig_file);
+		/* Limit no_populatable aging to once per try_to_shrink_lruvec() */
+		no_populatable = allow_no_populatable_age &&
+			((nr[LRU_GEN_ANON] && nr[LRU_GEN_ANON] == orig_anon) ||
+			(nr[LRU_GEN_FILE] && nr[LRU_GEN_FILE] == orig_file));
+		allow_no_populatable_age &= !no_populatable;
+		max_seq = READ_ONCE((lruvec)->lrugen.max_seq);
 		if (should_run_aging(lruvec, max_seq, sc, swappiness, no_populatable)) {
 			trace_mm_mglru_aging(lruvec, sc, swappiness);
 			if (try_to_inc_max_seq(lruvec, max_seq, swappiness, false))
 				need_rotate = true;
 			should_age = true;
 		}
+		if (!scanned)
+			break;
 
 		cond_resched();
 	}
@@ -5965,6 +5982,7 @@ static ssize_t lru_gen_seq_write(struct
 		.may_writepage = true,
 		.may_unmap = true,
 		.may_swap = true,
+		.priority = DEF_PRIORITY,
 		.reclaim_idx = MAX_NR_ZONES - 1,
 		.gfp_mask = GFP_KERNEL,
 		.proactive = true,
_