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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id bk23sm16516786qkb.3.2022.02.14.18.34.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Feb 2022 18:34:49 -0800 (PST) Date: Mon, 14 Feb 2022 18:34:46 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Andrew Morton cc: Michal Hocko , Vlastimil Babka , "Kirill A. Shutemov" , Matthew Wilcox , David Hildenbrand , Alistair Popple , Johannes Weiner , Rik van Riel , Suren Baghdasaryan , Yu Zhao , Greg Thelen , Shakeel Butt , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v2 09/13] mm/munlock: delete smp_mb() from __pagevec_lru_add_fn() In-Reply-To: <55a49083-37f9-3766-1de9-9feea7428ac@google.com> Message-ID: <28a7c6ff-6270-9060-8df0-862bdcaac366@google.com> References: <55a49083-37f9-3766-1de9-9feea7428ac@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Acked-by: Vlastimil Babka --- v2: same as v1. mm/swap.c | 37 +++++++++---------------------------- 1 file changed, 9 insertions(+), 28 deletions(-) diff --git a/mm/swap.c b/mm/swap.c index 682a03301a2c..3f770b1ea2c1 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -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); + folio_set_lru(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() + * 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. * - * - * 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. + * (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.) */ - folio_set_lru(folio); - smp_mb__after_atomic(); - if (folio_evictable(folio)) { if (was_unevictable) __count_vm_events(UNEVICTABLE_PGRESCUED, nr_pages); -- 2.34.1