Re: [External] Re: [PATCH v2] mm: memcontrol: fix swap undercounting in cgroup2

[Muchun Song]

On Wed, Feb 17, 2021 at 10:39 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> On Wed, Feb 17, 2021 at 3:09 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >
> > When pages are swapped in, the VM may retain the swap copy to avoid
> > repeated writes in the future. It's also retained if shared pages are
> > faulted back in some processes, but not in others. During that time we
> > have an in-memory copy of the page, as well as an on-swap copy. Cgroup1
> > and cgroup2 handle these overlapping lifetimes slightly differently
> > due to the nature of how they account memory and swap:
> >
> > Cgroup1 has a unified memory+swap counter that tracks a data page
> > regardless whether it's in-core or swapped out. On swapin, we transfer
> > the charge from the swap entry to the newly allocated swapcache page,
> > even though the swap entry might stick around for a while. That's why
> > we have a mem_cgroup_uncharge_swap() call inside mem_cgroup_charge().
> >
> > Cgroup2 tracks memory and swap as separate, independent resources and
> > thus has split memory and swap counters. On swapin, we charge the
> > newly allocated swapcache page as memory, while the swap slot in turn
> > must remain charged to the swap counter as long as its allocated too.
> >
> > The cgroup2 logic was broken by commit 2d1c498072de ("mm: memcontrol:
> > make swap tracking an integral part of memory control"), because it
> > accidentally removed the do_memsw_account() check in the branch inside
> > mem_cgroup_uncharge() that was supposed to tell the difference between
> > the charge transfer in cgroup1 and the separate counters in cgroup2.
> >
> > As a result, cgroup2 currently undercounts retained swap to varying
> > degrees: swap slots are cached up to 50% of the configured limit or
> > total available swap space; partially faulted back shared pages are
> > only limited by physical capacity. This in turn allows cgroups to
> > significantly overconsume their alloted swap space.
> >
> > Add the do_memsw_account() check back to fix this problem.
> >
> > Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # 5.8+
> > ---
> >  v2:
> >  - update commit log and add a comment to the code. Very thanks to Johannes.
> >
> >  mm/memcontrol.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index ed5cc78a8dbf..2efbb4f71d5f 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6771,7 +6771,19 @@ 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)) {
> > +       /*
> > +        * Cgroup1's unified memory+swap counter has been charged with the
> > +        * new swapcache page, finish the transfer by uncharging the swap
> > +        * slot. The swap slot would also get uncharged when it dies, but
> > +        * it can stick around indefinitely and we'd count the page twice
> > +        * the entire time.
> > +        *
> > +        * Cgroup2 has separate resource counters for memory and swap,
> > +        * so this is a non-issue here. Memory and swap charge lifetimes
> > +        * correspond 1:1 to page and swap slot lifetimes: we charge the
> > +        * page to memory here, and uncharge swap when the slot is freed.
> > +        */
> > +       if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && PageSwapCache(page)) {
>
> do_memsw_account() instead of !cgroup_subsys_on_dfl(memory_cgrp_subsys).

Thanks a lot. Will do.

>
> >                 swp_entry_t entry = { .val = page_private(page) };
> >                 /*
> >                  * The swap entry might not get freed for a long time,
> > --
> > 2.11.0
> >