On Sun, Oct 27, 2024 at 10:24:10PM -0700, Shakeel Butt wrote:
> On Fri, Oct 25, 2024 at 10:17:24AM GMT, Gregory Price 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.
> >
> > 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@xxxxxxxxxx>
> > Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> > Cc: stable@xxxxxxxxxxxxxxx
>
> Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
>
> This patch looks good for stable backports. For future I wonder if
> instead of migrate_pages(), the caller providing the isolated folios,
> manages the isolated stats (increments and decrements) similar to how
> reclaim does it.
>
Note that even if you provided the folios, you'd likely still end up in
migrate_pages_batch/migrate_folio_move and subsequently the same accounting
path. Probably there's some refactoring we can do to make the accounting
more obvious - it is very subtle here.
> > ---
> > 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
> >
