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Message-ID: <ZPXoZ2dmqXLkf6UV@dhcp22.suse.cz>
Date: Mon, 4 Sep 2023 16:23:35 +0200
From: Michal Hocko <mhocko@...e.com>
To: Liu Shixin <liushixin2@...wei.com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>,
Huang Ying <ying.huang@...el.com>,
Johannes Weiner <hannes@...xchg.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Shakeel Butt <shakeelb@...gle.com>,
Muchun Song <muchun.song@...ux.dev>,
Andrew Morton <akpm@...ux-foundation.org>,
wangkefeng.wang@...wei.com, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5] mm: vmscan: try to reclaim swapcache pages if no swap
space
On Wed 30-08-23 11:56:00, Liu Shixin 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.
>
> Fix it by checking number of swapcache pages in can_reclaim_anon_pages().
> If the number is not zero, return true either. Moreover, add a new bit
> swapcache_only in struct scan_control to skip isolating anon pages that
> are not swapcache when only swapcache pages can be reclaimed to accelerate
> reclaim efficiency.
Have you tested this also for the global reclaim? Am I just missing
something or this could seriously stall the reclaim in swapcache_only
mode if the swap cache pages cannot be dropped for some reason?
Also how big of a deal this is with somehow more realistic scenarios
with limits that are not so small?
> Link: https://lore.kernel.org/lkml/CAJD7tkZAfgncV+KbKr36=eDzMnT=9dZOT0dpMWcurHLr6Do+GA@mail.gmail.com/
> Signed-off-by: Liu Shixin <liushixin2@...wei.com>
> Tested-by: Yosry Ahmed <yosryahmed@...gle.com>
> Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
> Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
> v3->v4: Add describe and link about how to reproduce the problem.
> v4->v5: Add Reviewed-by and reproducer link.
>
> include/linux/swap.h | 6 ++++++
> mm/memcontrol.c | 8 ++++++++
> mm/vmscan.c | 29 +++++++++++++++++++++++++++--
> 3 files changed, 41 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 456546443f1f..0318e918bfa4 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -669,6 +669,7 @@ 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)
> @@ -691,6 +692,11 @@ 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 e8ca4bdcb03c..c465829db92b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7567,6 +7567,14 @@ 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 1080209a568b..e73e2df8828d 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;
>
> @@ -613,10 +616,20 @@ 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) {
> + 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) {
> + sc->swapcache_only = 1;
> + return true;
> + }
> }
>
> /*
> @@ -2280,6 +2293,19 @@ static bool skip_cma(struct folio *folio, struct scan_control *sc)
> }
> #endif
>
> +static bool skip_isolate(struct folio *folio, struct scan_control *sc,
> + enum lru_list lru)
> +{
> + if (folio_zonenum(folio) > sc->reclaim_idx)
> + return true;
> + if (skip_cma(folio, sc))
> + return true;
> + if (unlikely(sc->swapcache_only && !is_file_lru(lru) &&
> + !folio_test_swapcache(folio)))
> + return true;
> + return false;
> +}
> +
> /*
> * Isolating page from the lruvec to fill in @dst list by nr_to_scan times.
> *
> @@ -2326,8 +2352,7 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan,
> nr_pages = folio_nr_pages(folio);
> total_scan += nr_pages;
>
> - if (folio_zonenum(folio) > sc->reclaim_idx ||
> - skip_cma(folio, sc)) {
> + if (skip_isolate(folio, sc, lru)) {
> nr_skipped[folio_zonenum(folio)] += nr_pages;
> move_to = &folios_skipped;
> goto move;
> --
> 2.25.1
--
Michal Hocko
SUSE Labs
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