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Message-ID: <b00b1425-3942-f7f2-db9b-a45941e4fb44@huawei.com>
Date: Mon, 6 Nov 2023 14:43:39 +0800
From: Liu Shixin <liushixin2@...wei.com>
To: "Huang, Ying" <ying.huang@...el.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
On 2023/11/6 10:18, Huang, Ying wrote:
> 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.
OK, it‘s more appropriate, I will resend v8, thank you.
>> 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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