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

[David Rientjes <rientjes@google.com>]

On Thu, 16 Jul 2020, Shakeel Butt 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.
> 

Ah, so this adds to the motivation of providing the anon_reclaimable stat 
because the calculation becomes even more convoluted and completely based 
on the kernel implementation details for both lazyfree memory and deferred 
split queues.  Did you have a calculation in mind for 
memcg_anon_reclaimable()?