[RFC PATCH v5 14/14] mm/mglru: Avoid scan anon while swapcache is low for GFP_NOIO allocation

["Barry Song (Xiaomi)" <baohua@kernel.org>]

From: Bo Zhang <zhangbo56@xiaomi.com>

With GFP_NOIO, we cannot reclaim anon folios unless they are in
the swap cache. CLRU skips anon reclaim when the swap cache is
very low by commit 74d9aa0d8e98 ("mm: vmscan: avoid anon scanning
for GFP_NOIO with low swapcache"). Apply the same logic to MGLRU.

The new scan-budget-based mechanism makes this straightforward to
implement. Thanks also to Xiang Liu, who previously attempted to fix
this before we had the scan-budget-based model [1].

[1] https://lore.kernel.org/linux-mm/179248830836.1744016.8424217811286616919.noio-mglru-rfc-v2@bytedance.com/
Cc: Xiang Liu <liuxiang.277@bytedance.com>
Signed-off-by: Bo Zhang <zhangbo56@xiaomi.com>
Co-developed-by: Barry Song (Xiaomi) <baohua@kernel.org>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6d5a823667e0..d72a5aef3a0f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -379,13 +379,6 @@ static inline bool reclaimable_anon_is_low(struct mem_cgroup *memcg,
 	if (!sc || (sc->gfp_mask & __GFP_IO))
 		return false;
 
-	/*
-	 * FIXME: MGLRU doesn't fully respect can_reclaim_anon_pages() for the
-	 * scanning type, so only apply this to the traditional LRU for now.
-	 */
-	if (lru_gen_enabled())
-		return false;
-
 	if (memcg) {
 		struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
@@ -5061,26 +5054,23 @@ static void lru_gen_balance_scan(struct lruvec *lruvec, struct scan_control *sc,
 {
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 	u64 refault_rate[ANON_AND_FILE], anon_factor, file_factor;
-	u64 anon_scan, file_scan, total;
+	u64 anon_scan, file_scan;
 	u64 anon_weight, file_weight;
 	int size_bias;
 
-	total = nr[LRU_GEN_ANON] + nr[LRU_GEN_FILE];
-	if (!total)
-		return;
-
 	if (swappiness <= MIN_SWAPPINESS) {
 		nr[LRU_GEN_ANON] = 0;
 		nr[LRU_GEN_FILE] = min_t(u64, total_scan, nr[LRU_GEN_FILE]);
-		return;
 	}
 
 	if (swappiness == SWAPPINESS_ANON_ONLY) {
 		nr[LRU_GEN_FILE] = 0;
 		nr[LRU_GEN_ANON] = min_t(u64, total_scan, nr[LRU_GEN_ANON]);
-		return;
 	}
 
+	if (!nr[LRU_GEN_ANON] || !nr[LRU_GEN_FILE])
+		return;
+
 	for (int type = LRU_GEN_ANON; type <= LRU_GEN_FILE; type++) {
 		int hist = lru_hist_from_seq(READ_ONCE(lrugen->min_seq[type]));
 		/* start at MIN_LRU_BATCH to smooth the rate on gen start */
@@ -5306,6 +5296,8 @@ static void lru_gen_prepare_scan(struct lruvec *lruvec, struct scan_control *sc,
 	unsigned long total;
 
 	lruvec_evictable_size(lruvec, swappiness, nr_to_scan);
+	if (!can_reclaim_anon_pages(memcg, lruvec_pgdat(lruvec)->node_id, sc))
+		nr_to_scan[LRU_GEN_ANON] = 0;
 	total = nr_to_scan[LRU_GEN_ANON] + nr_to_scan[LRU_GEN_FILE];
 
 	/* Just scrape all pages calculated above if this memcg was deleted */
-- 
2.34.1