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

[Ge Yang <yangge1116@126.com>]

在 2024/7/30 15:45, David Hildenbrand 写道:
>>> 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.
> 

Right.

> 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.
> 
> 

When cpu0 runs folio_activate(), and cpu1 runs folio_put() concurrently, 
a possible bad scenario would like:

cpu0                                           cpu1

                                            folio_put_testzero(folio)
if (!folio_test_clear_lru(folio))// Seems folio shouldn't be accessed 

        return;
folio_get(folio);
                                             __folio_put(folio)
                                             __folio_clear_lru(folio)


Seems we should use folio_try_get(folio) instead of folio_get(folio).