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

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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).







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