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Message-ID: <87h6lzy68z.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Mon, 06 Nov 2023 10:18:04 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Liu Shixin <liushixin2@...wei.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Yosry Ahmed <yosryahmed@...gle.com>,
Sachin Sant <sachinp@...ux.ibm.com>,
Michal Hocko <mhocko@...e.com>,
Johannes Weiner <hannes@...xchg.org>,
Kefeng Wang <wangkefeng.wang@...wei.com>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7] mm: vmscan: try to reclaim swapcache pages if no
swap space
Liu Shixin <liushixin2@...wei.com> 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@mail.gmail.com/
> [2]. https://lore.kernel.org/linux-mm/CAJD7tkafz_2XAuqE8tGLPEcpLngewhUo=5US14PAtSM9tLBUQg@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>
> ---
> 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 | 36 ++++++++++++++++++++++++++++++++++--
> 3 files changed, 48 insertions(+), 2 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 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..a5e04291662f 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;
>
> @@ -602,6 +605,12 @@ static bool can_demote(int nid, struct scan_control *sc)
> return true;
> }
>
> +static void set_swapcache_mode(struct scan_control *sc, bool swapcache_only)
> +{
> + if (sc)
> + sc->swapcache_only = swapcache_only;
> +}
> +
I think that it's unnecessary to introduce a new function. I understand
that you want to reduce the code duplication. We can add
sc->swapcache_only = false;
at the beginning of can_reclaim_anon_pages() to reduce code duplication.
That can cover even more cases IIUC.
> static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
> int nid,
> struct scan_control *sc)
> @@ -611,12 +620,26 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
> * For non-memcg reclaim, is there
> * space in any swap device?
> */
> - if (get_nr_swap_pages() > 0)
> + if (get_nr_swap_pages() > 0) {
> + set_swapcache_mode(sc, false);
> return true;
> + }
> + /* Is there any swapcache pages to reclaim? */
> + if (total_swapcache_pages() > 0) {
> + set_swapcache_mode(sc, true);
> + return true;
> + }
> } else {
> /* Is the memcg below its swap limit? */
> - if (mem_cgroup_get_nr_swap_pages(memcg) > 0)
> + if (mem_cgroup_get_nr_swap_pages(memcg) > 0) {
> + set_swapcache_mode(sc, false);
> return true;
> + }
> + /* Is there any swapcache pages in memcg to reclaim? */
> + if (mem_cgroup_get_nr_swapcache_pages(memcg) > 0) {
> + set_swapcache_mode(sc, true);
> + return true;
> + }
> }
If can_demote() returns true, we shouldn't scan swapcache only.
--
Best Regards,
Huang, Ying
> /*
> @@ -2342,6 +2365,15 @@ 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))
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