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

[Yosry Ahmed]

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.

>
> --
> Best Regards,
> Huang, Ying