Re: [PATCH v8] mm: vmscan: try to reclaim swapcache pages if no swap space

[Huang, Ying]

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>
> ---
> 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          | 27 +++++++++++++++++++++++++++
>  3 files changed, 41 insertions(+)
>
> 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..5d5a169ec98c 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;
>  
> @@ -606,6 +609,9 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
>  					  int nid,
>  					  struct scan_control *sc)
>  {
> +	if (sc)
> +		sc->swapcache_only = 0;
> +
>  	if (memcg == NULL) {
>  		/*
>  		 * For non-memcg reclaim, is there
> @@ -613,10 +619,22 @@ 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) {
> +			if (sc)
> +				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) {
> +			if (sc)
> +				sc->swapcache_only = 1;
> +			return true;
> +		}
>  	}

I understand that this is only possible in theory.  But if can_demote()
== true, get_nr_swap_pages() == 0, total_swapcache_pages() > 0, we will
demote only anonymous pages in swapcache.  I think that this isn't
reasonable.  So, swapcache pages should be checked after can_demote()
checking.

--
Best Regards,
Huang, Ying

>  	/*
> @@ -2342,6 +2360,15 @@ 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))