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

[Shakeel Butt <shakeelb@google.com>]

On Wed, Feb 17, 2021 at 3:09 AM Muchun Song <songmuchun@bytedance.com> 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@bytedance.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Cc: stable@vger.kernel.org # 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).

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