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> --- v9->v10: Use per-node swapcache suggested by Yu Zhao. v8->v9: Move the swapcache check after can_demote() and refector can_reclaim_anon_pages() a bit. v7->v8: Reset swapcache_only at the beginning of can_reclaim_anon_pages(). 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. mm/vmscan.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 49 insertions(+), 1 deletion(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 506f8220c5fe..1fcc94717370 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -136,6 +136,9 @@ 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; @@ -308,10 +311,36 @@ static bool can_demote(int nid, struct scan_control *sc) return true; } +#ifdef CONFIG_SWAP +static bool can_reclaim_swapcache(struct mem_cgroup *memcg, int nid) +{ + struct pglist_data *pgdat = NODE_DATA(nid); + unsigned long nr_swapcache; + + if (!memcg) { + nr_swapcache = node_page_state(pgdat, NR_SWAPCACHE); + } else { + struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat); + + nr_swapcache = lruvec_page_state_local(lruvec, NR_SWAPCACHE); + } + + return nr_swapcache > 0; +} +#else +static bool can_reclaim_swapcache(struct mem_cgroup *memcg, int nid) +{ + return false; +} +#endif + static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg, int nid, struct scan_control *sc) { + if (sc) + sc->swapcache_only = 0; + if (memcg == NULL) { /* * For non-memcg reclaim, is there @@ -330,7 +359,17 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg, * * Can it be reclaimed from this node via demotion? */ - return can_demote(nid, sc); + if (can_demote(nid, sc)) + return true; + + /* Is there any swapcache pages to reclaim in this node? */ + if (can_reclaim_swapcache(memcg, nid)) { + if (sc) + sc->swapcache_only = 1; + return true; + } + + return false; } /* @@ -1642,6 +1681,15 @@ 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
