Re: [External] Re: [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2

[Muchun Song]

On Sat, Feb 13, 2021 at 2:57 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> CCing more folks.
>
> On Fri, Feb 12, 2021 at 9:14 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >
> > The swap charges the actual number of swap entries on cgroup v2.
> > If a swap cache page is charged successful, and then we uncharge
> > the swap counter. It is wrong on cgroup v2. Because the swap
> > entry is not freed.
> >
> > Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> What's the user visible impact of this change?

IIUC, I think that we cannot limit the swap to memory.swap.max
on cgroup v2.

  cd /sys/fs/cgroup/
  mkdir test
  cd test
  echo 8192 > memory.max
  echo 4096 > memory.swap.max

OK. Now we limit swap to 1 page and memory to 2 pages.
Firstly, we allocate 1 page from this memory cgroup and
swap this page to swap disk. We can see:

  memory.current: 0
  memory.swap.current: 1

Then we touch this page, we will swap in and charge
the swap cache page to the memory counter and uncharge
the swap counter.

  memory.current: 1
  memory.swap.current: 0 (but actually we use a swap entry)

Then we allocate another 1 page from this memory cgroup.

  memory.current: 2
  memory.swap.current: 0 (but actually we use a swap entry)

If we swap those 2 pages to swap disk. We can charge and swap
those 2 pages successfully. Right? Maybe I am wrong.

>
> One impact I can see is that without this patch meminfo's (SwapTotal -
> SwapFree) is larger than the sum of top level memory.swap.current.
> This change will reduce that gap.
>
> BTW what about per-cpu slots_ret cache? Should we call
> mem_cgroup_uncharge_swap() before putting in the cache after this
> change?
>
> > ---
> >  mm/memcontrol.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c737c8f05992..be6bc5044150 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6753,7 +6753,7 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
> >         memcg_check_events(memcg, page);
> >         local_irq_enable();
> >
> > -       if (PageSwapCache(page)) {
> > +       if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && PageSwapCache(page)) {
> >                 swp_entry_t entry = { .val = page_private(page) };
> >                 /*
> >                  * The swap entry might not get freed for a long time,
> > --
> > 2.11.0
> >