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mm/munlock: delete smp_mb() from __pagevec_lru_add_fn()

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My reading of comment on smp_mb__after_atomic() in __pagevec_lru_add_fn()
says that it can now be deleted; and that remains so when the next patch
is added.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
This commit is contained in:
Hugh Dickins 2022-02-14 18:34:46 -08:00 committed by Matthew Wilcox (Oracle)
parent c3096e6782
commit 2262ace607
1 changed files with 11 additions and 30 deletions
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41
mm/swap.c
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@ -1025,37 +1025,18 @@ static void __pagevec_lru_add_fn(struct folio *folio, struct lruvec *lruvec)
VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
/*
* A folio becomes evictable in two ways:
* 1) Within LRU lock [munlock_vma_page() and __munlock_pagevec()].
* 2) Before acquiring LRU lock to put the folio on the correct LRU
* and then
* a) do PageLRU check with lock [check_move_unevictable_pages]
* b) do PageLRU check before lock [clear_page_mlock]
*
* (1) & (2a) are ok as LRU lock will serialize them. For (2b), we need
* following strict ordering:
*
* #0: __pagevec_lru_add_fn #1: clear_page_mlock
*
* folio_set_lru() folio_test_clear_mlocked()
* smp_mb() // explicit ordering // above provides strict
* // ordering
* folio_test_mlocked() folio_test_lru()
*
*
* if '#1' does not observe setting of PG_lru by '#0' and
* fails isolation, the explicit barrier will make sure that
* folio_evictable check will put the folio on the correct
* LRU. Without smp_mb(), folio_set_lru() can be reordered
* after folio_test_mlocked() check and can make '#1' fail the
* isolation of the folio whose mlocked bit is cleared (#0 is
* also looking at the same folio) and the evictable folio will
* be stranded on an unevictable LRU.
*/
folio_set_lru(folio);
smp_mb__after_atomic();
/*
* Is an smp_mb__after_atomic() still required here, before
* folio_evictable() tests PageMlocked, to rule out the possibility
* of stranding an evictable folio on an unevictable LRU? I think
* not, because munlock_page() only clears PageMlocked while the LRU
* lock is held.
*
* (That is not true of __page_cache_release(), and not necessarily
* true of release_pages(): but those only clear PageMlocked after
* put_page_testzero() has excluded any other users of the page.)
*/
if (folio_evictable(folio)) {
if (was_unevictable)
__count_vm_events(UNEVICTABLE_PGRESCUED, nr_pages);