[PATCH -next v6] mm: vmscan: retry folios written back while isolated for traditional LRU

[Chen Ridong]

From: Chen Ridong <chenridong@xxxxxxxxxx>

The page reclaim isolates a batch of folios from the tail of one of the
LRU lists and works on those folios one by one.  For a suitable
swap-backed folio, if the swap device is async, it queues that folio for
writeback.  After the page reclaim finishes an entire batch, it puts back
the folios it queued for writeback to the head of the original LRU list.

In the meantime, the page writeback flushes the queued folios also by
batches.  Its batching logic is independent from that of the page reclaim.
For each of the folios it writes back, the page writeback calls
folio_rotate_reclaimable() which tries to rotate a folio to the tail.

folio_rotate_reclaimable() only works for a folio after the page reclaim
has put it back.  If an async swap device is fast enough, the page
writeback can finish with that folio while the page reclaim is still
working on the rest of the batch containing it.  In this case, that folio
will remain at the head and the page reclaim will not retry it before
reaching there.

The commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back
while isolated") only fixed the issue for mglru. However, this issue
also exists in the traditional active/inactive LRU. This issue will be
worse if THP is split, which makes the list longer and needs longer time
to finish a batch of folios reclaim.

This issue should be fixed in the same way for the traditional LRU.
Therefore, the common logic was extracted to the 'find_folios_written_back'
function firstly, which is then reused in the 'shrink_inactive_list'
function. Finally, retry reclaiming those folios that may have missed the
rotation for traditional LRU.

Link: https://lore.kernel.org/linux-kernel/20241010081802.290893-1-chenridong@xxxxxxxxxxxxxxx/
Link: https://lore.kernel.org/linux-kernel/CAGsJ_4zqL8ZHNRZ44o_CC69kE7DBVXvbZfvmQxMGiFqRxqHQdA@xxxxxxxxxxxxxx/
Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
---

v5->v6:
 - fix compile error(implicit declaration of function 'lru_gen_distance')
   when CONFIG_LRU_GEN is disable.
 - rename 'is_retried' to is_retrying suggested by Barry Song.

v5: https://lore.kernel.org/linux-kernel/CAGsJ_4x3Aj7wieK1FQKQC4Vbz5N+1dExs=Q70KQt-whS1dMxpw@xxxxxxxxxxxxxx/

 include/linux/mm_inline.h |   5 ++
 mm/vmscan.c               | 108 ++++++++++++++++++++++++--------------
 2 files changed, 75 insertions(+), 38 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 3fcf5fa797fe..07b2fda6fafa 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -342,6 +342,11 @@ static inline void folio_migrate_refs(struct folio *new, struct folio *old)
 {
 
 }
+
+static inline int lru_gen_distance(struct folio *folio, bool reclaiming)
+{
+	return -1;
+}
 #endif /* CONFIG_LRU_GEN */
 
 static __always_inline
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 39886f435ec5..701716306f8b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -283,6 +283,39 @@ static void set_task_reclaim_state(struct task_struct *task,
 	task->reclaim_state = rs;
 }
 
+/**
+ * find_folios_written_back - Find and move the written back folios to a new list.
+ * @list: filios list
+ * @clean: the written back folios list
+ * @is_retrying: whether the list is retrying.
+ */
+static inline void find_folios_written_back(struct list_head *list,
+		struct list_head *clean, bool is_retrying)
+{
+	struct folio *folio;
+	struct folio *next;
+
+	list_for_each_entry_safe_reverse(folio, next, list, lru) {
+		if (!folio_evictable(folio)) {
+			list_del(&folio->lru);
+			folio_putback_lru(folio);
+			continue;
+		}
+
+		/* retry folios that may have missed folio_rotate_reclaimable() */
+		if (!is_retrying && !folio_test_active(folio) && !folio_mapped(folio) &&
+		    !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
+			list_move(&folio->lru, clean);
+			continue;
+		}
+
+		/* don't add rejected folios to the oldest generation */
+		if (lru_gen_enabled() && !lru_gen_distance(folio, false))
+			set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
+	}
+
+}
+
 /*
  * flush_reclaim_state(): add pages reclaimed outside of LRU-based reclaim to
  * scan_control->nr_reclaimed.
@@ -1959,14 +1992,18 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 		enum lru_list lru)
 {
 	LIST_HEAD(folio_list);
+	LIST_HEAD(clean_list);
 	unsigned long nr_scanned;
-	unsigned int nr_reclaimed = 0;
+	unsigned int nr_reclaimed, total_reclaimed = 0;
+	unsigned int nr_pageout = 0;
+	unsigned int nr_unqueued_dirty = 0;
 	unsigned long nr_taken;
 	struct reclaim_stat stat;
 	bool file = is_file_lru(lru);
 	enum vm_event_item item;
 	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
 	bool stalled = false;
+	bool is_retrying = false;
 
 	while (unlikely(too_many_isolated(pgdat, file, sc))) {
 		if (stalled)
@@ -2000,22 +2037,47 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 	if (nr_taken == 0)
 		return 0;
 
+retry:
 	nr_reclaimed = shrink_folio_list(&folio_list, pgdat, sc, &stat, false);
 
+	sc->nr.dirty += stat.nr_dirty;
+	sc->nr.congested += stat.nr_congested;
+	sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
+	sc->nr.writeback += stat.nr_writeback;
+	sc->nr.immediate += stat.nr_immediate;
+	total_reclaimed += nr_reclaimed;
+	nr_pageout += stat.nr_pageout;
+	nr_unqueued_dirty += stat.nr_unqueued_dirty;
+
+	trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
+			nr_scanned, nr_reclaimed, &stat, sc->priority, file);
+
+	find_folios_written_back(&folio_list, &clean_list, is_retrying);
+
 	spin_lock_irq(&lruvec->lru_lock);
 	move_folios_to_lru(lruvec, &folio_list);
 
 	__mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(),
 					stat.nr_demoted);
-	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
 	item = PGSTEAL_KSWAPD + reclaimer_offset();
 	if (!cgroup_reclaim(sc))
 		__count_vm_events(item, nr_reclaimed);
 	__count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
 	__count_vm_events(PGSTEAL_ANON + file, nr_reclaimed);
+
+	if (!list_empty(&clean_list)) {
+		list_splice_init(&clean_list, &folio_list);
+		is_retrying = true;
+		spin_unlock_irq(&lruvec->lru_lock);
+		goto retry;
+	}
+	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
 	spin_unlock_irq(&lruvec->lru_lock);
+	sc->nr.taken += nr_taken;
+	if (file)
+		sc->nr.file_taken += nr_taken;
 
-	lru_note_cost(lruvec, file, stat.nr_pageout, nr_scanned - nr_reclaimed);
+	lru_note_cost(lruvec, file, nr_pageout, nr_scanned - total_reclaimed);
 
 	/*
 	 * If dirty folios are scanned that are not queued for IO, it
@@ -2028,7 +2090,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 	 * the flushers simply cannot keep up with the allocation
 	 * rate. Nudge the flusher threads in case they are asleep.
 	 */
-	if (stat.nr_unqueued_dirty == nr_taken) {
+	if (nr_unqueued_dirty == nr_taken) {
 		wakeup_flusher_threads(WB_REASON_VMSCAN);
 		/*
 		 * For cgroupv1 dirty throttling is achieved by waking up
@@ -2043,18 +2105,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
 	}
 
-	sc->nr.dirty += stat.nr_dirty;
-	sc->nr.congested += stat.nr_congested;
-	sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
-	sc->nr.writeback += stat.nr_writeback;
-	sc->nr.immediate += stat.nr_immediate;
-	sc->nr.taken += nr_taken;
-	if (file)
-		sc->nr.file_taken += nr_taken;
-
-	trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
-			nr_scanned, nr_reclaimed, &stat, sc->priority, file);
-	return nr_reclaimed;
+	return total_reclaimed;
 }
 
 /*
@@ -4585,12 +4636,10 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
 	int reclaimed;
 	LIST_HEAD(list);
 	LIST_HEAD(clean);
-	struct folio *folio;
-	struct folio *next;
 	enum vm_event_item item;
 	struct reclaim_stat stat;
 	struct lru_gen_mm_walk *walk;
-	bool skip_retry = false;
+	bool is_retrying = false;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
@@ -4616,24 +4665,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
 			scanned, reclaimed, &stat, sc->priority,
 			type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
 
-	list_for_each_entry_safe_reverse(folio, next, &list, lru) {
-		if (!folio_evictable(folio)) {
-			list_del(&folio->lru);
-			folio_putback_lru(folio);
-			continue;
-		}
-
-		/* retry folios that may have missed folio_rotate_reclaimable() */
-		if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
-		    !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
-			list_move(&folio->lru, &clean);
-			continue;
-		}
-
-		/* don't add rejected folios to the oldest generation */
-		if (!lru_gen_distance(folio, false))
-			set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
-	}
+	find_folios_written_back(&list, &clean, is_retrying);
 
 	spin_lock_irq(&lruvec->lru_lock);
 
@@ -4656,7 +4688,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
 	list_splice_init(&clean, &list);
 
 	if (!list_empty(&list)) {
-		skip_retry = true;
+		is_retrying = true;
 		goto retry;
 	}
 
-- 
2.34.1