mm/lruvec: preemptively free dead folios during lru_add drain

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.

PG_active may be set on a batched folio as well as PG_unevictable (via
migration path).  Since filtered folios bypass the normal lru_add()
cleanup, both flags must be cleared before freeing.

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%).

Note that tests were performed using classic LRU.

Link: https://lore.kernel.org/20260425053417.351146-1-jp.kobryn@linux.dev
Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swap.c

@@ -160,14 +160,42 @@ 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.
+		 *
+		 * We're bypassing normal cleanup. Clear flags that are not
+		 * applicable to dead folios.
+		 *
+		 * 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_clear_active(folio);
+			__folio_clear_unevictable(folio);
+			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 +204,13 @@ 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 +999,10 @@ 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;