Re: [PATCH] mm/migrate: Putback split folios when numa hint migration fails

[Peter Xu]

On Tue, Jul 09, 2024 at 09:48:54AM +0800, Baolin Wang wrote:
> 
> 
> On 2024/7/9 05:55, Peter Xu wrote:
> > This issue is not from any report yet, but by code observation only.
> > 
> > This is yet another fix besides Hugh's patch [1] but on relevant code path,
> > where eager split of folio can happen if the folio is already on deferred
> > list during a folio migration.
> > 
> > Here the issue is NUMA path (migrate_misplaced_folio()) may start to
> > encounter such folio split now even with MR_NUMA_MISPLACED hint applied.
> > Then when migrate_pages() didn't migrate all the folios, it's possible the
> > split small folios be put onto the list instead of the original folio.
> > Then putting back only the head page won't be enough.
> > 
> > Fix it by putting back all the folios on the list.
> > 
> > [1] https://lore.kernel.org/all/46c948b4-4dd8-6e03-4c7b-ce4e81cfa536@xxxxxxxxxx/
> > 
> > Cc: Zi Yan <ziy@xxxxxxxxxx>
> > Cc: Yang Shi <shy828301@xxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Fixes: 7262f208ca68 ("mm/migrate: split source folio if it is on deferred split list")
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> 
> Good catch. With fixing the building issue pointed by Andrew, please feel
> free to add:
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> 
> > 
> > Don't need to copy stable if this can still hit 6.10..  Only smoke tested.
> > ---
> >   mm/migrate.c | 10 ++--------
> >   1 file changed, 2 insertions(+), 8 deletions(-)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index e10d2445fbd8..20da2595527a 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2615,14 +2615,8 @@ int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma,
> >   	nr_remaining = migrate_pages(&migratepages, alloc_misplaced_dst_folio,
> >   				     NULL, node, MIGRATE_ASYNC,
> >   				     MR_NUMA_MISPLACED, &nr_succeeded);
> > -	if (nr_remaining) {
> > -		if (!list_empty(&migratepages)) {
> > -			list_del(&folio->lru);
> > -			node_stat_mod_folio(folio, NR_ISOLATED_ANON +
> > -					folio_is_file_lru(folio), -nr_pages);
> > -			folio_putback_lru(folio);
> > -		}
> > -	}
> > +	if (nr_remaining && !list_empty(&migratepages))
> 
> Nit: you can drop the '!list_empty(&migratepages)' validation, since
> putback_movable_pages() can handle this unusual case.

Sure, considering that it should normally be !empty when the first check
passed.

Though to make this simple for now, I assume we can keep what has been
queued in Andrew's tree.  It isn't so bad either to double check the list
to avoid a function call if possible, I think.

Thanks for the comment,

-- 
Peter Xu