On 2023/11/6 14:57, Huang, Ying wrote: > 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 Thanks for your advise, that sounds more reasonable. I will modify it soon. > >> /* >> @@ -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)) > . >
