From 0c4f8ed498cea1e2beebf7aa3d198fa381264f88 Mon Sep 17 00:00:00 2001 From: Joanne Koong Date: Mon, 14 Apr 2025 15:22:09 -0700 Subject: [PATCH] mm: skip folio reclaim in legacy memcg contexts for deadlockable mappings Currently in shrink_folio_list(), reclaim for folios under writeback falls into 3 different cases: 1) Reclaim is encountering an excessive number of folios under writeback and this folio has both the writeback and reclaim flags set 2) Dirty throttling is enabled (this happens if reclaim through cgroup is not enabled, if reclaim through cgroupv2 memcg is enabled, or if reclaim is on the root cgroup), or if the folio is not marked for immediate reclaim, or if the caller does not have __GFP_FS (or __GFP_IO if it's going to swap) set 3) Legacy cgroupv1 encounters a folio that already has the reclaim flag set and the caller did not have __GFP_FS (or __GFP_IO if swap) set In cases 1) and 2), we activate the folio and skip reclaiming it while in case 3), we wait for writeback to finish on the folio and then try to reclaim the folio again. In case 3, we wait on writeback because cgroupv1 does not have dirty folio throttling, as such this is a mitigation against the case where there are too many folios in writeback with nothing else to reclaim. If a filesystem (eg fuse) may deadlock due to reclaim waiting on writeback, then the filesystem needs to add inefficient messy workarounds to prevent this. To improve the performance of these filesystems, this commit adds two things: a) a AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM mapping flag that filesystems may set to indicate that reclaim should not wait on writeback b) if legacy memcg encounters a folio with this AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM flag set (eg case 3), the folio will be activated and skip reclaim (eg default to behavior in case 2) instead. Signed-off-by: Joanne Koong Reviewed-by: Shakeel Butt Reviewed-by: Jeff Layton Acked-by: David Hildenbrand Reviewed-by: Jingbo Xu Signed-off-by: Miklos Szeredi --- include/linux/pagemap.h | 11 +++++++++++ mm/vmscan.c | 12 +++++++++--- 2 files changed, 20 insertions(+), 3 deletions(-) diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 26baa78f1ca7d..e19ff3183bbf0 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -210,6 +210,7 @@ enum mapping_flags { AS_STABLE_WRITES = 7, /* must wait for writeback before modifying folio contents */ AS_INACCESSIBLE = 8, /* Do not attempt direct R/W access to the mapping */ + AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9, /* Bits 16-25 are used for FOLIO_ORDER */ AS_FOLIO_ORDER_BITS = 5, AS_FOLIO_ORDER_MIN = 16, @@ -335,6 +336,16 @@ static inline bool mapping_inaccessible(struct address_space *mapping) return test_bit(AS_INACCESSIBLE, &mapping->flags); } +static inline void mapping_set_writeback_may_deadlock_on_reclaim(struct address_space *mapping) +{ + set_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags); +} + +static inline bool mapping_writeback_may_deadlock_on_reclaim(struct address_space *mapping) +{ + return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags); +} + static inline gfp_t mapping_gfp_mask(struct address_space * mapping) { return mapping->gfp_mask; diff --git a/mm/vmscan.c b/mm/vmscan.c index b620d74b0f66e..8fc180fbd9443 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1187,8 +1187,10 @@ retry: * 2) Global or new memcg reclaim encounters a folio that is * not marked for immediate reclaim, or the caller does not * have __GFP_FS (or __GFP_IO if it's simply going to swap, - * not to fs). In this case mark the folio for immediate - * reclaim and continue scanning. + * not to fs), or the folio belongs to a mapping where + * waiting on writeback during reclaim may lead to a deadlock. + * In this case mark the folio for immediate reclaim and + * continue scanning. * * Require may_enter_fs() because we would wait on fs, which * may not have submitted I/O yet. And the loop driver might @@ -1213,6 +1215,8 @@ retry: * takes to write them to disk. */ if (folio_test_writeback(folio)) { + mapping = folio_mapping(folio); + /* Case 1 above */ if (current_is_kswapd() && folio_test_reclaim(folio) && @@ -1223,7 +1227,9 @@ retry: /* Case 2 above */ } else if (writeback_throttling_sane(sc) || !folio_test_reclaim(folio) || - !may_enter_fs(folio, sc->gfp_mask)) { + !may_enter_fs(folio, sc->gfp_mask) || + (mapping && + mapping_writeback_may_deadlock_on_reclaim(mapping))) { /* * This is slightly racy - * folio_end_writeback() might have -- 2.47.2