Re: [PATCH] mm/migrate: fix stale partially_mapped arg to deferred_split_folio()

[David Hildenbrand (Arm)]

On 4/1/26 10:41, Deepanshu Kartikey wrote:
> In migrate_folio_move(), src_partially_mapped is sampled from the source
> folio before move_to_new_folio() is called:
> 
>   if (folio_order(src) > 1 &&
>       !data_race(list_empty(&src->_deferred_list))) {
>           src_deferred_split = true;
>           src_partially_mapped = folio_test_partially_mapped(src);
>   }
> 
> A concurrent thread can unmap pages from the source folio between this
> read and the actual migration, making the sampled value stale.

Trying to make sense of this.

In migrate_folio_move() don't we have the folio completely unmapped
because there are only migration entries referencing the folio?

See migrate_folio_unmap(), where we check !folio_mapped().

Why should we suddenly have mapped folio here? Something is off.

Unmapping a migration entry will not involve rmap code and not mess with
the partially-mapped flag.

-- 
Cheers,

David