Re: [PATCH v1] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems

[Mina Almasry <almasrymina@google.com>]

On Fri, Dec 2, 2022 at 8:14 PM Wei Xu <weixugc@google.com> wrote:
>
> On Fri, Dec 2, 2022 at 5:11 PM Mina Almasry <almasrymina@google.com> wrote:
> >
> > commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
> > reclaim"") enabled demotion in memcg reclaim, which is the right thing
> > to do, however, I suspect it introduced a regression in the behavior of
> > try_to_free_mem_cgroup_pages().
> >
> > The callers of try_to_free_mem_cgroup_pages() expect it to attempt to
> > reclaim - not demote - nr_pages from the cgroup. I.e. the memory usage
> > of the cgroup should reduce by nr_pages. The callers expect
> > try_to_free_mem_cgroup_pages() to also return the number of pages
> > reclaimed, not demoted.
> >
> > However, what try_to_free_mem_cgroup_pages() actually does is it
> > unconditionally counts demoted pages as reclaimed pages. So in practice
> > when it is called it will often demote nr_pages and return the number of
> > demoted pages to the caller. Demoted pages don't lower the memcg usage,
> > and so I think try_to_free_mem_cgroup_pages() is not actually doing what
> > the callers want it to do.
> >
> > I suspect various things work suboptimally on memory systems or don't
> > work at all due to this:
> >
> > - memory.high enforcement likely doesn't work (it just demotes nr_pages
> >   instead of lowering the memcg usage by nr_pages).
> > - try_charge_memcg() will keep retrying the charge while
> >   try_to_free_mem_cgroup_pages() is just demoting pages and not actually
> >   making any room for the charge.
> > - memory.reclaim has a wonky interface. It advertises to the user it
> >   reclaims the provided amount but it will actually demote that amount.
> >
> > There may be more effects to this issue.
> >
> > To fix these issues I propose shrink_folio_list() to only count pages
> > demoted from inside of sc->nodemask to outside of sc->nodemask as
> > 'reclaimed'.
> >
> > For callers such as reclaim_high() or try_charge_memcg() that set
> > sc->nodemask to NULL, try_to_free_mem_cgroup_pages() will try to
> > actually reclaim nr_pages and return the number of pages reclaimed. No
> > demoted pages would count towards the nr_pages requirement.
> >
> > For callers such as memory_reclaim() that set sc->nodemask,
> > try_to_free_mem_cgroup_pages() will free nr_pages from that nodemask
> > with either reclaim or demotion.
> >
> > Tested this change using memory.reclaim interface. With this change,
> >
> >         echo "1m" > memory.reclaim
> >
> > Will cause freeing of 1m of memory from the cgroup regardless of the
> > demotions happening inside.
> >
> >         echo "1m nodes=0" > memory.reclaim
> >
> > Will cause freeing of 1m of node 0 by demotion if a demotion target is
> > available, and by reclaim if no demotion target is available.
> >
> > Signed-off-by: Mina Almasry <almasrymina@google.com>
> >
> > ---
> >
> > This is developed on top of mm-unstable largely because I need the
> > memory.reclaim nodes= arg to test it properly.
> > ---
> >  mm/vmscan.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 2b42ac9ad755..8f6e993b870d 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1653,6 +1653,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >         LIST_HEAD(free_folios);
> >         LIST_HEAD(demote_folios);
> >         unsigned int nr_reclaimed = 0;
> > +       unsigned int nr_demoted = 0;
> >         unsigned int pgactivate = 0;
> >         bool do_demote_pass;
> >         struct swap_iocb *plug = NULL;
> > @@ -2085,7 +2086,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >         /* 'folio_list' is always empty here */
> >
> >         /* Migrate folios selected for demotion */
> > -       nr_reclaimed += demote_folio_list(&demote_folios, pgdat);
> > +       nr_demoted = demote_folio_list(&demote_folios, pgdat);
> > +
> > +       /*
> > +        * Only count demoted folios as reclaimed if we demoted them from
> > +        * inside of the nodemask to outside of the nodemask, hence reclaiming
> > +        * pages in the nodemask.
> > +        */
> > +       if (sc->nodemask && node_isset(pgdat->node_id, *sc->nodemask) &&
> > +           !node_isset(next_demotion_node(pgdat->node_id), *sc->nodemask))
>
> next_demotion_node() is just the first demotion target node. Demotion
> can fall back to other allowed target nodes returned by
> node_get_allowed_targets().  When the page is demoted to a fallback
> node and this fallback node is in sc->nodemask, nr_demoted should not
> be added into nr_reclaimed, either.
>

Thanks for reviewing Wei, I did indeed miss this.

> One way to address this issue is to pass sc->nodemask into
> demote_folio_list() and exclude sc->nodemask from the allowed target
> demotion nodes.
>

This makes sense to me. Applied this change and uploaded v2:
https://lore.kernel.org/linux-mm/20221204093008.2620459-1-almasrymina@google.com/T/#u

> > +               nr_reclaimed += nr_demoted;
> > +
> >         /* Folios that could not be demoted are still in @demote_folios */
> >         if (!list_empty(&demote_folios)) {
> >                 /* Folios which weren't demoted go back on @folio_list */
> > --
> > 2.39.0.rc0.267.gcb52ba06e7-goog