Re: [PATCH v7] mm: vmscan: try to reclaim swapcache pages if no swap space

[Liu Shixin]

On 2023/11/6 10:18, 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>
>> ---
>> 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))
> .
>