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

["Huang, Ying" <ying.huang@intel.com>]

Gregory Price <gourry@gourry.net> writes:

> 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

I think that it may be better to mention the different behavior of LRU
and MGLRU.  But that's not a big deal, change it again only if you think
it's necessary.

--
Best Regards,
Huang, Ying

> 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
>> >