Liu Shixin <liushixin2@xxxxxxxxxx> writes: > 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> > --- > 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. > > include/linux/swap.h | 6 ++++++ > mm/memcontrol.c | 8 ++++++++ > mm/vmscan.c | 27 +++++++++++++++++++++++++++ > 3 files changed, 41 insertions(+) > > 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..5d5a169ec98c 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; > > @@ -606,6 +609,9 @@ 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 > @@ -613,10 +619,22 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg, > */ > if (get_nr_swap_pages() > 0) > return true; > + /* Is there any swapcache pages to reclaim? */ > + if (total_swapcache_pages() > 0) { > + if (sc) > + sc->swapcache_only = 1; > + return true; > + } > } else { > /* Is the memcg below its swap limit? */ > if (mem_cgroup_get_nr_swap_pages(memcg) > 0) > return true; > + /* Is there any swapcache pages in memcg to reclaim? */ > + if (mem_cgroup_get_nr_swapcache_pages(memcg) > 0) { > + if (sc) > + sc->swapcache_only = 1; > + return true; > + } > } I understand that this is only possible in theory. But if can_demote() == true, get_nr_swap_pages() == 0, total_swapcache_pages() > 0, we will demote only anonymous pages in swapcache. I think that this isn't reasonable. So, swapcache pages should be checked after can_demote() checking. -- Best Regards, Huang, Ying > /* > @@ -2342,6 +2360,15 @@ 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))
