Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to LRU batch

[David Hildenbrand <david@redhat.com>]

>> Looking at this in more detail, I wonder if we can turn that to
>>
>> if (!folio_test_clear_lru(folio))
>>       return;
>> folio_get(folio);
>>
>> In all cases? The caller must hold a reference, so this should be fine.
>>
> 
> Seems the caller madvise_free_pte_range(...), calling
> folio_mark_lazyfree(...), doesn't hold a reference on folio.
> 

If that would be the case and the folio could get freed concurrently, 
the folio_get(folio) would be completely broken.

In madvise_free_pte_range() we hold the PTL, so the folio cannot get 
freed concurrently.

folio_get() is only allowed when we are sure the folio cannot get freed 
concurrently, because we know there is a reference that cannot go away.


-- 
Cheers,

David / dhildenb