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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/1/26 12:10, David Hildenbrand (Arm) wrote:
> 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.
> 

Okay, concluding that the above reasoning is all wrong.

Let's discuss it with Lance's proposal, which makes more sense.

https://lore.kernel.org/r/20260401085932.20945-1-lance.yang@linux.dev

-- 
Cheers,

David