When spaces of swap devices are exhausted, only file pages can be reclaimed. But there are still some swapcache pages in anon lru list. This can lead to a premature out-of-memory. The problem is found with such step: Firstly, set a 9MB disk swap space, then create a cgroup with 10MB memory limit, then runs an program to allocates about 15MB memory. The problem occurs occasionally, which may need about 100 times [1]. Fix it by checking number of swapcache pages in can_reclaim_anon_pages(). If the number is not zero, return true and set swapcache_only to 1. When scan anon lru list in swapcache_only mode, non-swapcache pages will be skipped to isolate in order to accelerate reclaim efficiency. However, in swapcache_only mode, the scan count still increased when scan non-swapcache pages because there are large number of non-swapcache pages and rare swapcache pages in swapcache_only mode, and if the non-swapcache is skipped and do not count, the scan of pages in isolate_lru_folios() can eventually lead to hung task, just as Sachin reported [2]. By the way, since there are enough times of memory reclaim before OOM, it is not need to isolate too much swapcache pages in one times. [1]. https://lore.kernel.org/lkml/CAJD7tkZAfgncV+KbKr36=eDzMnT=9dZOT0dpMWcurHLr6Do+GA@xxxxxxxxxxxxxx/ [2]. https://lore.kernel.org/linux-mm/CAJD7tkafz_2XAuqE8tGLPEcpLngewhUo=5US14PAtSM9tLBUQg@xxxxxxxxxxxxxx/ Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx> Tested-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx> Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> --- v6->v7: Reset swapcache_only to zero after there are swap spaces. v5->v6: Fix NULL pointing derefence and hung task problem reported by Sachin. include/linux/swap.h | 6 ++++++ mm/memcontrol.c | 8 ++++++++ mm/vmscan.c | 36 ++++++++++++++++++++++++++++++++++-- 3 files changed, 48 insertions(+), 2 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index f6dd6575b905..3ba146ae7cf5 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -659,6 +659,7 @@ static inline void mem_cgroup_uncharge_swap(swp_entry_t entry, unsigned int nr_p } extern long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg); +extern long mem_cgroup_get_nr_swapcache_pages(struct mem_cgroup *memcg); extern bool mem_cgroup_swap_full(struct folio *folio); #else static inline void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry) @@ -681,6 +682,11 @@ static inline long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg) return get_nr_swap_pages(); } +static inline long mem_cgroup_get_nr_swapcache_pages(struct mem_cgroup *memcg) +{ + return total_swapcache_pages(); +} + static inline bool mem_cgroup_swap_full(struct folio *folio) { return vm_swap_full(); diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 5b009b233ab8..29e34c06ca83 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -7584,6 +7584,14 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg) return nr_swap_pages; } +long mem_cgroup_get_nr_swapcache_pages(struct mem_cgroup *memcg) +{ + if (mem_cgroup_disabled()) + return total_swapcache_pages(); + + return memcg_page_state(memcg, NR_SWAPCACHE); +} + bool mem_cgroup_swap_full(struct folio *folio) { struct mem_cgroup *memcg; diff --git a/mm/vmscan.c b/mm/vmscan.c index 6f13394b112e..a5e04291662f 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -137,6 +137,9 @@ struct scan_control { /* Always discard instead of demoting to lower tier memory */ unsigned int no_demotion:1; + /* Swap space is exhausted, only reclaim swapcache for anon LRU */ + unsigned int swapcache_only:1; + /* Allocation order */ s8 order; @@ -602,6 +605,12 @@ static bool can_demote(int nid, struct scan_control *sc) return true; } +static void set_swapcache_mode(struct scan_control *sc, bool swapcache_only) +{ + if (sc) + sc->swapcache_only = swapcache_only; +} + static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg, int nid, struct scan_control *sc) @@ -611,12 +620,26 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg, * For non-memcg reclaim, is there * space in any swap device? */ - if (get_nr_swap_pages() > 0) + if (get_nr_swap_pages() > 0) { + set_swapcache_mode(sc, false); return true; + } + /* Is there any swapcache pages to reclaim? */ + if (total_swapcache_pages() > 0) { + set_swapcache_mode(sc, true); + return true; + } } else { /* Is the memcg below its swap limit? */ - if (mem_cgroup_get_nr_swap_pages(memcg) > 0) + if (mem_cgroup_get_nr_swap_pages(memcg) > 0) { + set_swapcache_mode(sc, false); return true; + } + /* Is there any swapcache pages in memcg to reclaim? */ + if (mem_cgroup_get_nr_swapcache_pages(memcg) > 0) { + set_swapcache_mode(sc, true); + return true; + } } /* @@ -2342,6 +2365,15 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan, */ scan += nr_pages; + /* + * Count non-swapcache too because the swapcache pages may + * be rare and it takes too much times here if not count + * the non-swapcache pages. + */ + if (unlikely(sc->swapcache_only && !is_file_lru(lru) && + !folio_test_swapcache(folio))) + goto move; + if (!folio_test_lru(folio)) goto move; if (!sc->may_unmap && folio_mapped(folio)) -- 2.25.1
