Re: [patch] mm, memcg: provide an anon_reclaimable stat

[Shakeel Butt <shakeelb@google.com>]

On Thu, Jul 16, 2020 at 1:58 PM David Rientjes <rientjes@google.com> wrote:
>
> Userspace can lack insight into the amount of memory that can be reclaimed
> from a memcg based on values from memory.stat.  Two specific examples:
>
>  - Lazy freeable memory (MADV_FREE) that are clean anonymous pages on the
>    inactive file LRU that can be quickly reclaimed under memory pressure
>    but otherwise shows up as mapped anon in memory.stat, and
>
>  - Memory on deferred split queues (thp) that are compound pages that can
>    be split and uncharged from the memcg under memory pressure, but
>    otherwise shows up as charged anon LRU memory in memory.stat.
>
> Both of this anonymous usage is also charged to memory.current.
>
> Userspace can currently derive this information but it depends on kernel
> implementation details for how this memory is handled for the purposes of
> reclaim (anon on inactive file LRU or unmapped anon on the LRU).
>
> For the purposes of writing portable userspace code that does not need to
> have insight into the kernel implementation for reclaimable memory, this
> exports a stat that reveals the amount of anonymous memory that can be
> reclaimed and uncharged from the memcg to start new applications.
>
> As the kernel implementation evolves for memory that can be reclaimed
> under memory pressure, this stat can be kept consistent.
>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  6 +++++
>  mm/memcontrol.c                         | 31 +++++++++++++++++++++++++
>  2 files changed, 37 insertions(+)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1296,6 +1296,12 @@ PAGE_SIZE multiple when read back.
>                 Amount of memory used in anonymous mappings backed by
>                 transparent hugepages
>
> +         anon_reclaimable
> +               The amount of charged anonymous memory that can be reclaimed
> +               under memory pressure without swap.  This currently includes
> +               lazy freeable memory (MADV_FREE) and compound pages that can be
> +               split and uncharged.
> +
>           inactive_anon, active_anon, inactive_file, active_file, unevictable
>                 Amount of memory, swap-backed and filesystem-backed,
>                 on the internal memory management lists used by the
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1350,6 +1350,32 @@ static bool mem_cgroup_wait_acct_move(struct mem_cgroup *memcg)
>         return false;
>  }
>
> +/*
> + * Returns the amount of anon memory that is charged to the memcg that is
> + * reclaimable under memory pressure without swap, in pages.
> + */
> +static unsigned long memcg_anon_reclaimable(struct mem_cgroup *memcg)
> +{
> +       long deferred, lazyfree;
> +
> +       /*
> +        * Deferred pages are charged anonymous pages that are on the LRU but
> +        * are unmapped.  These compound pages are split under memory pressure.
> +        */
> +       deferred = max_t(long, memcg_page_state(memcg, NR_ACTIVE_ANON) +
> +                              memcg_page_state(memcg, NR_INACTIVE_ANON) -
> +                              memcg_page_state(memcg, NR_ANON_MAPPED), 0);

Please note that the NR_ANON_MAPPED does not include tmpfs memory but
NR_[IN]ACTIVE_ANON does include the tmpfs.

> +       /*
> +        * Lazyfree pages are charged clean anonymous pages that are on the file
> +        * LRU and can be reclaimed under memory pressure.
> +        */
> +       lazyfree = max_t(long, memcg_page_state(memcg, NR_ACTIVE_FILE) +
> +                              memcg_page_state(memcg, NR_INACTIVE_FILE) -
> +                              memcg_page_state(memcg, NR_FILE_PAGES), 0);

Similarly NR_FILE_PAGES includes tmpfs memory but NR_[IN]ACTIVE_FILE does not.