[PATCH] mm/lruvec: preemptively free dead folios during lru_add drain

["JP Kobryn (Meta)" <jp.kobryn@linux.dev>]

Of all observable lruvec lock contention in our fleet, we find that ~24%
occurs when dead folios are present in lru_add batches at drain time. This
is wasteful in the sense that the folio is added to the LRU just to be
immediately removed via folios_put_refs(), incurring two unnecessary lock
acquisitions.

Eliminate this overhead by preemptively cleaning up dead folios before they
make it into the LRU. Use folio_ref_freeze() to filter folios whose only
remaining refcount is the batch ref. When dead folios are found, move them
off the add batch and onto a temporary batch to be freed.

During A/B testing on one of our prod instagram workloads (high-frequency
short-lived requests), the patch intercepted almost all dead folios before
they entered the LRU. Data collected using the mm_lru_insertion tracepoint
shows the effectiveness of the patch:

Per-host LRU add averages at 95% CPU load
(60 hosts each side, 3 x 60s intervals)

            dead folios/min  total folios/min   dead %
unpatched:        1,297,785        19,341,986  6.7097%
patched:                 14        19,039,996  0.0001%

Within this workload, we save ~2.6M lock acquisitions per minute per host
as a result.

System-wide memory stats improved on the patched side also at 95% CPU load:
 - direct reclaim scanning reduced 7%
 - allocation stalls reduced 5.2%
 - compaction stalls reduced 12.3%
 - page frees reduced 4.9%

No regressions were observed in requests served per second or request tail
latency (p99). Both metrics showed directional improvement at higher CPU
utilization (comparing 85% to 95%).

Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
---
 mm/swap.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/mm/swap.c b/mm/swap.c
index 5cc44f0de9877..71607b0ce3d18 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -160,13 +160,36 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
 	int i;
 	struct lruvec *lruvec = NULL;
 	unsigned long flags = 0;
+	struct folio_batch free_fbatch;
+	bool is_lru_add = (move_fn == lru_add);
+
+	/*
+	 * If we're adding to the LRU, preemptively filter dead folios. Use
+	 * this dedicated folio batch for temp storage and deferred cleanup.
+	 */
+	if (is_lru_add)
+		folio_batch_init(&free_fbatch);
 
 	for (i = 0; i < folio_batch_count(fbatch); i++) {
 		struct folio *folio = fbatch->folios[i];
 
 		/* block memcg migration while the folio moves between lru */
-		if (move_fn != lru_add && !folio_test_clear_lru(folio))
+		if (!is_lru_add && !folio_test_clear_lru(folio))
+			continue;
+
+		/*
+		 * Filter dead folios by moving them from the add batch to the temp
+		 * batch for freeing after this loop.
+		 *
+		 * Since the folio may be part of a huge page, unqueue from
+		 * deferred split list to avoid a dangling list entry.
+		 */
+		if (is_lru_add && folio_ref_freeze(folio, 1)) {
+			folio_unqueue_deferred_split(folio);
+			fbatch->folios[i] = NULL;
+			folio_batch_add(&free_fbatch, folio);
 			continue;
+		}
 
 		folio_lruvec_relock_irqsave(folio, &lruvec, &flags);
 		move_fn(lruvec, folio);
@@ -176,6 +199,13 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
 
 	if (lruvec)
 		lruvec_unlock_irqrestore(lruvec, flags);
+
+	/* Cleanup filtered dead folios. */
+	if (is_lru_add) {
+		mem_cgroup_uncharge_folios(&free_fbatch);
+		free_unref_folios(&free_fbatch);
+	}
+
 	folios_put(fbatch);
 }
 
@@ -964,6 +994,10 @@ void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
 		struct folio *folio = folios->folios[i];
 		unsigned int nr_refs = refs ? refs[i] : 1;
 
+		/* Folio batch entry may have been preemptively removed during drain. */
+		if (!folio)
+			continue;
+
 		if (is_huge_zero_folio(folio))
 			continue;
 
-- 
2.52.0