Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Gregory Price <gourry@gourry.net>]

On Mon, Oct 28, 2024 at 01:45:48PM -0700, Yang Shi wrote:
> On Fri, Oct 25, 2024 at 7:17 AM Gregory Price <gourry@gourry.net> wrote:
> >
> > When numa balancing is enabled with demotion, vmscan will call
> > migrate_pages when shrinking LRUs.  Successful demotions will
> > cause node vmstat numbers to double-decrement, leading to an
> > imbalanced page count.  The result is dmesg output like such:
> >
> > $ cat /proc/sys/vm/stat_refresh
> >
> > [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> > [77383.088417] vmstat_refresh: nr_isolated_file -899642
> >
> > This negative value may impact compaction and reclaim throttling.
> >
> > The double-decrement occurs in the migrate_pages path:
> >
> > caller to shrink_folio_list decrements the count
> >   shrink_folio_list
> >     demote_folio_list
> >       migrate_pages
> >         migrate_pages_batch
> >           migrate_folio_move
> >             migrate_folio_done
> >               mod_node_page_state(-ve) <- second decrement
> >
> > This path happens for SUCCESSFUL migrations, not failures. Typically
> > callers to migrate_pages are required to handle putback/accounting for
> > failures, but this is already handled in the shrink code.
> 
> AFAIK, MGLRU doesn't dec/inc this counter, so it is not
> double-decrement for MGLRU. Maybe "imbalance update" is better?
> Anyway, it is just a nit. I'd suggest capturing the MGLRU case in the
> commit log too.
>

Gotcha, so yeah saying it's an imbalance fix is more accurate.

So more accurate changelog is:


[PATCH] vmscan,migrate: fix page count imbalance on node stats when demoting pages

When numa balancing is enabled with demotion, vmscan will call
migrate_pages when shrinking LRUs.  migrate_pages will decrement the
the node's isolated page count, leading to an imbalanced count when
invoked from (MG)LRU code.

The result is dmesg output like such:

$ cat /proc/sys/vm/stat_refresh

[77383.088417] vmstat_refresh: nr_isolated_anon -103212
[77383.088417] vmstat_refresh: nr_isolated_file -899642

This negative value may impact compaction and reclaim throttling.

The following path produces the decrement:

shrink_folio_list
  demote_folio_list
    migrate_pages
      migrate_pages_batch
        migrate_folio_move
          migrate_folio_done
            mod_node_page_state(-ve) <- decrement

This path happens for SUCCESSFUL migrations, not failures. Typically
callers to migrate_pages are required to handle putback/accounting for
failures, but this is already handled in the shrink code.

When accounting for migrations, instead do not decrement the count
when the migration reason is MR_DEMOTION. As of v6.11, this demotion
logic is the only source of MR_DEMOTION.


> >
> > Signed-off-by: Gregory Price <gourry@gourry.net>
> > Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> > Cc: stable@vger.kernel.org
> 
> Thanks for catching this. Reviewed-by: Yang Shi <shy828301@gmail.com>
> 
> > ---
> >  mm/migrate.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 923ea80ba744..e3aac274cf16 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
> >          * not accounted to NR_ISOLATED_*. They can be recognized
> >          * as __folio_test_movable
> >          */
> > -       if (likely(!__folio_test_movable(src)))
> > +       if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
> >                 mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
> >                                     folio_is_file_lru(src), -folio_nr_pages(src));
> >
> > --
> > 2.43.0
> >