From: Ridong Chen The page reclaim isolates a batch of folios from the tail of an LRU list and works on them one by one. For a suitable swap-backed folio on an async swap device, it queues the folio for writeback and, after finishing the batch, puts the folio back to the head of the original LRU list. Meanwhile the page writeback flushes the queued folios in its own, independent batches. For each folio it writes back it calls folio_rotate_reclaimable(), which tries to rotate the folio to the LRU tail. But folio_rotate_reclaimable() only takes effect once the folio has been put back by reclaim. If the async swap device is fast enough, the writeback can complete a folio while reclaim is still working on the rest of the batch that contains it. In that case the folio stays near the head and reclaim will not revisit it before wrapping around, causing a cold/hot inversion: a clean, written-back folio that should be a prime reclaim candidate is kept ahead of hotter folios. commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back while isolated") addressed this for MGLRU only. The traditional active/inactive LRU has the same problem, reported at [1]. A reproducer is available at [2]. Rather than re-reclaiming those folios (which would drop the swap cache that may still be useful for a future hit [4]), restore the rotation that was missed: when move_folios_to_lru() puts a folio back, add it to the LRU tail if it looks like it missed folio_rotate_reclaimable() (inactive, not mapped, not dirty and not under writeback). A referenced folio is left at the head so it still gets a second chance, and a folio with an unexpected reference (e.g. a GUP or speculative pin) is left at the head because it cannot be reclaimed yet anyway. A new do_rotate parameter gates this so it only applies on the reclaim put-back path (shrink_inactive_list()), not on shrink_active_list() where the list order is already deliberate. This approach was suggested by Barry Song [3]. Only the traditional LRU is handled here. MGLRU already retries such folios via its own clean-list retry pass in evict_folios(), so it is left unchanged. The same do_rotate scheme could later replace that retry pass to unify both LRUs, which is left for a follow-up. Test result with [2]: Without patch: cat memory.usage_in_bytes 1073700864 cat memory.memsw.usage_in_bytes 1413124096 free -h total used free Mem: 1.6Gi 1.2Gi 299Mi Swap: 1.0Gi 678Mi 346Mi With patch: cat memory.usage_in_bytes 1071140864 cat memory.memsw.usage_in_bytes 1413423104 free -h total used free Mem: 1.6Gi 1.2Gi 322Mi Swap: 1.0Gi 328Mi 695Mi After applying the patch, the difference between memory.memsw.usage_in_bytes and memory.usage_in_bytes is close to the swap "used" value reported by 'free -h'. [1] https://lore.kernel.org/linux-kernel/20241010081802.290893-1-chenridong@huaweicloud.com/ [2] https://lore.kernel.org/lkml/46037a37-4cf6-448e-a94b-30a4d16e8814@linux.dev/ [3] https://lore.kernel.org/lkml/CAGsJ_4zwP3_+EYY5Ug9EJ+yD1UdxsBSGr25u8s1K3u_i7LH3Zg@mail.gmail.com/ [4] https://lore.kernel.org/linux-mm/20260911121341.178028-1-alex@ghiti.fr/ Suggested-by: Barry Song Signed-off-by: Ridong Chen --- v1 -> v2: - skip referenced folios (FOLIOREF_KEEP) and unexpectedly pinned folios when rotating to the LRU tail. - add test result to the commit message. mm/vmscan.c | 24 ++++++++++++++++++------ 1 file changed, 18 insertions(+), 6 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index e200ce3eb056..91295070ca33 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1971,7 +1971,7 @@ static bool too_many_isolated(struct pglist_data *pgdat, int file, * * Note: The caller must not hold any lruvec lock. */ -static unsigned int move_folios_to_lru(struct list_head *list) +static unsigned int move_folios_to_lru(struct list_head *list, bool do_rotate) { int nr_pages, nr_moved = 0; struct lruvec *lruvec = NULL; @@ -2018,7 +2018,19 @@ static unsigned int move_folios_to_lru(struct list_head *list) continue; } - lruvec_add_folio(lruvec, folio); + /* + * Put clean, unreferenced and unpinned folios that may have + * missed folio_rotate_reclaimable() at the tail to avoid + * cold/hot inversion. + */ + if (do_rotate && !folio_test_active(folio) && !folio_mapped(folio) && + !folio_test_dirty(folio) && !folio_test_writeback(folio) && + !folio_test_referenced(folio) && + folio_ref_count(folio) == folio_expected_ref_count(folio)) + lruvec_add_folio_tail(lruvec, folio); + else + lruvec_add_folio(lruvec, folio); + nr_pages = folio_nr_pages(folio); nr_moved += nr_pages; if (folio_test_active(folio)) @@ -2135,7 +2147,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan, nr_reclaimed = shrink_folio_list(&folio_list, pgdat, sc, &stat, false, lruvec_memcg(lruvec)); - move_folios_to_lru(&folio_list); + move_folios_to_lru(&folio_list, true); mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(sc), stat.nr_demoted); @@ -2246,8 +2258,8 @@ static void shrink_active_list(unsigned long nr_to_scan, /* * Move folios back to the lru list. */ - nr_activate = move_folios_to_lru(&l_active); - nr_deactivate = move_folios_to_lru(&l_inactive); + nr_activate = move_folios_to_lru(&l_active, false); + nr_deactivate = move_folios_to_lru(&l_inactive, false); count_vm_events(PGDEACTIVATE, nr_deactivate); count_memcg_events(lruvec_memcg(lruvec), PGDEACTIVATE, nr_deactivate); @@ -5115,7 +5127,7 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec, folio_set_active(folio); } - move_folios_to_lru(&list); + move_folios_to_lru(&list, false); walk = current->reclaim_state->mm_walk; if (walk && walk->batched) { -- 2.34.1