On Tue, Nov 14, 2023 at 9:24 PM Liu Shixin <liushixin2@xxxxxxxxxx> wrote: > > 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> > --- > 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. > > include/linux/swap.h | 6 ++++++ > mm/memcontrol.c | 8 ++++++++ > mm/vmscan.c | 47 ++++++++++++++++++++++++++++++++------------ > 3 files changed, 48 insertions(+), 13 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 774bd6e21e27..a76ec540d4a3 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -7865,6 +7865,14 @@ 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 506f8220c5fe..62a1c75f74ad 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; > > @@ -312,25 +315,34 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg, > int nid, > struct scan_control *sc) > { > - if (memcg == NULL) { > - /* > - * For non-memcg reclaim, is there > - * space in any swap device? > - */ > - if (get_nr_swap_pages() > 0) > - return true; > - } else { > - /* Is the memcg below its swap limit? */ > - if (mem_cgroup_get_nr_swap_pages(memcg) > 0) > - return true; > - } > + if (sc) > + sc->swapcache_only = 0; > + > + /* > + * For non-memcg reclaim, is there space in any swap device? > + * Or is the memcg below its swap limit? > + */ > + if ((!memcg && get_nr_swap_pages() > 0) || > + (memcg && mem_cgroup_get_nr_swap_pages(memcg) > 0)) > + return true; > > /* > * The page can not be swapped. > * > * 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? */ > + if ((!memcg && total_swapcache_pages() > 0) || > + (memcg && mem_cgroup_get_nr_swapcache_pages(memcg) > 0)) { The above can return false positives if there are multiple nodes, so it needs to be per node or lruvec, i.e., node_page_state() or lruvec_page_state_local().
