From: JP Kobryn (Meta) Date: Sat, 25 Apr 2026 05:34:17 +0000 (-0700) Subject: mm/lruvec: preemptively free dead folios during lru_add drain X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9669b87065a6fe96198f3df2c3d125c5f5c1f210;p=thirdparty%2Fkernel%2Flinux.git 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) Reviewed-by: Matthew Wilcox (Oracle) Acked-by: Shakeel Butt Acked-by: Michal Hocko Cc: Axel Rasmussen Cc: Baoquan He Cc: Barry Song Cc: Chris Li Cc: Johannes Weiner Cc: Kairui Song Cc: Kemeng Shi Cc: Nhat Pham Cc: Rik van Riel Cc: Wei Xu Cc: Yuanchu Xie Signed-off-by: Andrew Morton --- diff --git a/mm/swap.c b/mm/swap.c index 5cc44f0de987..2dd84813f4dd 100644 --- a/mm/swap.c +++ b/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;