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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> > Cc: stable@xxxxxxxxxxxxxxx
>
> Thanks for catching this. Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
>
> > ---
> > 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
> >
