Re: [PATCH v2] mm: vmscan: reclaim anon pages if there are swapcache pages

[Huang, Ying]

Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:

> On Thu, Aug 24, 2023 at 1:51 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
>>
>> > On Mon, Aug 21, 2023 at 6:54 PM Liu Shixin <liushixin2@xxxxxxxxxx> 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.
>> >>
>> >> This problem can be fixed by checking number of swapcache pages in
>> >> can_reclaim_anon_pages(). For memcg v2, there are swapcache stat that can
>> >> be used directly. For memcg v1, use total_swapcache_pages() instead, which
>> >> may not accurate but can solve the problem.
>> >
>> > Interesting find. I wonder if we really don't have any handling of
>> > this situation.
>> >
>> >>
>> >> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
>> >> ---
>> >>  include/linux/swap.h |  6 ++++++
>> >>  mm/memcontrol.c      |  8 ++++++++
>> >>  mm/vmscan.c          | 12 ++++++++----
>> >>  3 files changed, 22 insertions(+), 4 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..3e578f41023e 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() || do_memsw_account())
>> >> +               return total_swapcache_pages();
>> >> +
>> >> +       return memcg_page_state(memcg, NR_SWAPCACHE);
>> >> +}
>> >
>> > Is there a reason why we cannot use NR_SWAPCACHE for cgroup v1? Isn't
>> > that being maintained regardless of cgroup version? It is not exposed
>> > in cgroup v1's memory.stat, but I don't think there is a reason we
>> > can't do that -- if only to document that it is being used with cgroup
>> > v1.
>> >
>> >
>> >> +
>> >>  bool mem_cgroup_swap_full(struct folio *folio)
>> >>  {
>> >>         struct mem_cgroup *memcg;
>> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> >> index 7c33c5b653ef..bcb6279cbae7 100644
>> >> --- a/mm/vmscan.c
>> >> +++ b/mm/vmscan.c
>> >> @@ -609,13 +609,17 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
>> >>         if (memcg == NULL) {
>> >>                 /*
>> >>                  * For non-memcg reclaim, is there
>> >> -                * space in any swap device?
>> >> +                * space in any swap device or swapcache pages?
>> >>                  */
>> >> -               if (get_nr_swap_pages() > 0)
>> >> +               if (get_nr_swap_pages() + total_swapcache_pages() > 0)
>> >>                         return true;
>> >>         } else {
>> >> -               /* Is the memcg below its swap limit? */
>> >> -               if (mem_cgroup_get_nr_swap_pages(memcg) > 0)
>> >> +               /*
>> >> +                * Is the memcg below its swap limit or is there swapcache
>> >> +                * pages can be freed?
>> >> +                */
>> >> +               if (mem_cgroup_get_nr_swap_pages(memcg) +
>> >> +                   mem_cgroup_get_nr_swapcache_pages(memcg) > 0)
>> >>                         return true;
>> >>         }
>> >
>> > I wonder if it would be more efficient to set a bit in struct
>> > scan_control if we only are out of swap spaces but have swap cache
>> > pages, and only isolate anon pages that are in the swap cache, instead
>> > of isolating random anon pages. We may end up isolating pages that are
>> > not in the swap cache for a few iterations and wasting cycles.
>>
>> Scanning swap cache directly will make the code more complex.  IIUC, the
>> possibility for the swap device to be used up isn't high.  If so, I
>> prefer the simpler implementation as that in this series.
>
> I did not mean that, sorry if I wasn't clear. I meant to set a bit in
> struct scan_control, and then in isolate_lru_folios() for anon lrus,
> we can skip isolating folios that are not in the swapcache if that bit
> is set.
>
> My main concern was that if we have a few pages in the swapcache we
> may end up wasting cycles scanning through a lot of other anonymous
> pages until we reach them. If that's too much complexity that's
> understandable.

Sorry, I misunderstood your idea.  This sounds reasonable to me.  We can
check swap space and swap cache in isolate_lru_folios(), either in
isolate_lru_folios() directly, or via a bit in scan_control.

--
Best Regards,
Huang, Ying