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

[Huang, Ying]

Gregory Price <gourry@xxxxxxxxxx> writes:

> 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.

I agree with Shakeel here.  It's better for the caller who isolates the
folios to increase and decrease the isolation counter.  And yes, some
refactoring is required.

--
Best Regards,
Huang, Ying

>> > ---
>> >  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
>> >