On Thu, Jul 16, 2020 at 1:58 PM David Rientjes <rientjes@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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.
