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

[Wei Xu]

On Fri, Dec 2, 2022 at 5:11 PM Mina Almasry <almasrymina@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> ---
>
> 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.

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.

> +               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