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Received: from 30.97.56.72(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0W9iE.E5_1719912314) by smtp.aliyun-inc.com; Tue, 02 Jul 2024 17:25:14 +0800 Message-ID: Date: Tue, 2 Jul 2024 17:25:13 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH hotfix] mm: fix crashes from deferred split racing folio migration To: Hugh Dickins , Andrew Morton Cc: Nhat Pham , Yang Shi , Zi Yan , Barry Song , Kefeng Wang , David Hildenbrand , Matthew Wilcox , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <29c83d1a-11ca-b6c9-f92e-6ccb322af510@google.com> From: Baolin Wang In-Reply-To: <29c83d1a-11ca-b6c9-f92e-6ccb322af510@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 22FC740015 X-Stat-Signature: t45fw1agceoaqfsia373a8sqqd9of8z4 X-Rspam-User: X-HE-Tag: 1719912318-213687 X-HE-Meta: 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 163xX/zw IxAxm8WOZjskSpy0EclmqnIeJ9TKlGDx13zWigK5jXGXDKo4rIYCmc/EtKAaHmHMS9xprorUdszgxh+M/5kpHz9AJRr7/bSVhcfN4yNLYAKmlwI3HVIe/s6a3+EvsFBNgKZBHFvuxjwTFO2Ji8PGxfJakW1+mUB9uQSJosnzsN3yBQtGZy6O1vycvSXFKKuAGVcp36jJkZTyEU9PBhnIFI2L5J6O3J/No4d15ValX3+Qe/yChPFxNDls6UtMJa5L1KqLfjxWY9pKYUk7Jwg4AKgeVE1efJPdFdEaBNIxP/ukmJuCl0nwMzMbEt+q5xaTLqAgOsfB3wWiyMasLxks7SGksKZWajHQpnfJ6suK5V4apDyOoex2MiebYGuXU+zcONEUZI6UcrKC8bB/w0lWVErLI9g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2024/7/2 15:40, Hugh Dickins wrote: > Even on 6.10-rc6, I've been seeing elusive "Bad page state"s (often on > flags when freeing, yet the flags shown are not bad: PG_locked had been > set and cleared??), and VM_BUG_ON_PAGE(page_ref_count(page) == 0)s from > deferred_split_scan()'s folio_put(), and a variety of other BUG and WARN > symptoms implying double free by deferred split and large folio migration. > > 6.7 commit 9bcef5973e31 ("mm: memcg: fix split queue list crash when large > folio migration") was right to fix the memcg-dependent locking broken in > 85ce2c517ade ("memcontrol: only transfer the memcg data for migration"), > but missed a subtlety of deferred_split_scan(): it moves folios to its own > local list to work on them without split_queue_lock, during which time > folio->_deferred_list is not empty, but even the "right" lock does nothing > to secure the folio and the list it is on. > > Fortunately, deferred_split_scan() is careful to use folio_try_get(): so > folio_migrate_mapping() can avoid the race by folio_undo_large_rmappable() > while the old folio's reference count is temporarily frozen to 0 - adding > such a freeze in the !mapping case too (originally, folio lock and > unmapping and no swap cache left an anon folio unreachable, so no freezing > was needed there: but the deferred split queue offers a way to reach it). Thanks Hugh. But after reading your analysis, I am concerned that the folio_undo_large_rmappable() and deferred_split_scan() may still encounter a race condition with the local list, even with your patch. Suppose folio A has already been queued into the local list in deferred_split_scan() by thread A, but fails to 'folio_trylock' and then releases the reference count. At the same time, folio A can be frozen by another thread B in folio_migrate_mapping(). In such a case, folio_undo_large_rmappable() would remove folio A from the local list without *any* lock protection, creating a race condition with the local list iteration in deferred_split_scan(). Anyway, I think this patch can still fix some possible races. Feel free to add: Reviewed-by: Baolin Wang > > Fixes: 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration") > Signed-off-by: Hugh Dickins > Cc: stable@vger.kernel.org > --- > This patch against 6.10-rc6: Kefeng has commits in the mm-tree which > which will need adjustment to go over this, but we can both check the > result. I have wondered whether just reverting 85ce2c517ade and its > subsequent fixups would be better: but that would be a bigger job, > and probably not the right choice. > > mm/memcontrol.c | 11 ----------- > mm/migrate.c | 13 +++++++++++++ > 2 files changed, 13 insertions(+), 11 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 71fe2a95b8bd..8f2f1bb18c9c 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -7823,17 +7823,6 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new) > > /* Transfer the charge and the css ref */ > commit_charge(new, memcg); > - /* > - * If the old folio is a large folio and is in the split queue, it needs > - * to be removed from the split queue now, in case getting an incorrect > - * split queue in destroy_large_folio() after the memcg of the old folio > - * is cleared. > - * > - * In addition, the old folio is about to be freed after migration, so > - * removing from the split queue a bit earlier seems reasonable. > - */ > - if (folio_test_large(old) && folio_test_large_rmappable(old)) > - folio_undo_large_rmappable(old); > old->memcg_data = 0; > } > > diff --git a/mm/migrate.c b/mm/migrate.c > index 20cb9f5f7446..a8c6f466e33a 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -415,6 +415,15 @@ int folio_migrate_mapping(struct address_space *mapping, > if (folio_ref_count(folio) != expected_count) > return -EAGAIN; > > + /* Take off deferred split queue while frozen and memcg set */ > + if (folio_test_large(folio) && > + folio_test_large_rmappable(folio)) { > + if (!folio_ref_freeze(folio, expected_count)) > + return -EAGAIN; > + folio_undo_large_rmappable(folio); > + folio_ref_unfreeze(folio, expected_count); > + } > + > /* No turning back from here */ > newfolio->index = folio->index; > newfolio->mapping = folio->mapping; > @@ -433,6 +442,10 @@ int folio_migrate_mapping(struct address_space *mapping, > return -EAGAIN; > } > > + /* Take off deferred split queue while frozen and memcg set */ > + if (folio_test_large(folio) && folio_test_large_rmappable(folio)) > + folio_undo_large_rmappable(folio); > + > /* > * Now we know that no one else is looking at the folio: > * no turning back from here.