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From: Matthew Wilcox <willy@infradead.org>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kirill@shutemov.name, Nicholas Piggin <npiggin@gmail.com>
Subject: Race between freeing and waking page
Date: Thu, 27 Aug 2020 13:20:19 +0100	[thread overview]
Message-ID: <20200827122019.GC14765@casper.infradead.org> (raw)
In-Reply-To: <alpine.LSU.2.11.2008262301240.4405@eggly.anvils>

On Thu, Aug 27, 2020 at 12:01:00AM -0700, Hugh Dickins wrote:
> It was a crash from checking PageWaiters on a Tail in wake_up_page(),
> called from end_page_writeback(), from ext4_finish_bio(): yet the
> page a tail of a shmem huge page.  Linus's wake_up_page_bit() changes?
> No, I don't think so.  It seems to me that once end_page_writeback()
> has done its test_clear_page_writeback(), it has no further hold on
> the struct page, which could be reused as part of a compound page
> by the time of wake_up_page()'s PageWaiters check.  But I probably
> need to muse on that for longer.

I think you're right.  Example:

truncate_inode_pages_range()
pagevec_lookup_entries()
lock_page()

--- ctx switch ---

ext4_finish_bio()
end_page_writeback()
test_clear_page_writeback()

--- ctx switch ---

wait_on_page_writeback() <- noop
truncate_inode_page()
unlock_page()
pagevec_release()

... page can now be allocated

--- ctx switch ---

wake_up_page()
PageWaiters then has that check for PageTail.

This isn't unique to ext4; the iomap completion path behaves the exact
same way.  The thing is, this is a harmless race.  It seems unnecessary
for anybody here to incur the overhead of adding a page ref to be sure
the page isn't reallocated.  We don't want to wake up the waiters before
clearing the bit in question.

I'm tempted to suggest this:

 static void wake_up_page(struct page *page, int bit)
 {
-       if (!PageWaiters(page))
+       if (PageTail(page) || !PageWaiters(page))
                return;
        wake_up_page_bit(page, bit);

which only adds an extra read to the struct page that we were going to
access anyway.  Even that seems unnecessary though; PageWaiters is
going to be clear.  Maybe we can just change the PF policy from
PF_ONLY_HEAD to PF_ANY.  I don't think it's critical that we have this
check.

Nick?


  reply	other threads:[~2020-08-27 12:20 UTC|newest]

Thread overview: 102+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-24 12:54 [PATCH v18 00/32] per memcg lru_lock Alex Shi
2020-08-24 12:54 ` [PATCH v18 01/32] mm/memcg: warning on !memcg after readahead page charged Alex Shi
2020-08-24 12:54 ` [PATCH v18 02/32] mm/memcg: bail out early from swap accounting when memcg is disabled Alex Shi
2020-08-24 12:54 ` [PATCH v18 03/32] mm/thp: move lru_add_page_tail func to huge_memory.c Alex Shi
2020-08-24 12:54 ` [PATCH v18 04/32] mm/thp: clean up lru_add_page_tail Alex Shi
2020-08-24 12:54 ` [PATCH v18 05/32] mm/thp: remove code path which never got into Alex Shi
2020-08-24 12:54 ` [PATCH v18 06/32] mm/thp: narrow lru locking Alex Shi
2020-09-10 13:49   ` Matthew Wilcox
2020-09-11  3:37     ` Alex Shi
2020-09-13 15:27       ` Matthew Wilcox
2020-09-19  1:00         ` Hugh Dickins
2020-08-24 12:54 ` [PATCH v18 07/32] mm/swap.c: stop deactivate_file_page if page not on lru Alex Shi
2020-08-24 12:54 ` [PATCH v18 08/32] mm/vmscan: remove unnecessary lruvec adding Alex Shi
2020-08-24 12:54 ` [PATCH v18 09/32] mm/page_idle: no unlikely double check for idle page counting Alex Shi
2020-08-24 12:54 ` [PATCH v18 10/32] mm/compaction: rename compact_deferred as compact_should_defer Alex Shi
2020-08-24 12:54 ` [PATCH v18 11/32] mm/memcg: add debug checking in lock_page_memcg Alex Shi
2020-08-24 12:54 ` [PATCH v18 12/32] mm/memcg: optimize mem_cgroup_page_lruvec Alex Shi
2020-08-24 12:54 ` [PATCH v18 13/32] mm/swap.c: fold vm event PGROTATED into pagevec_move_tail_fn Alex Shi
2020-08-24 12:54 ` [PATCH v18 14/32] mm/lru: move lru_lock holding in func lru_note_cost_page Alex Shi
2020-08-24 12:54 ` [PATCH v18 15/32] mm/lru: move lock into lru_note_cost Alex Shi
2020-09-21 21:36   ` Hugh Dickins
2020-09-21 22:03     ` Hugh Dickins
2020-09-22  3:39       ` Alex Shi
2020-09-22  3:38     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 16/32] mm/lru: introduce TestClearPageLRU Alex Shi
2020-09-21 23:16   ` Hugh Dickins
2020-09-22  3:53     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 17/32] mm/compaction: do page isolation first in compaction Alex Shi
2020-09-21 23:49   ` Hugh Dickins
2020-09-22  4:57     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 18/32] mm/thp: add tail pages into lru anyway in split_huge_page() Alex Shi
2020-08-24 12:54 ` [PATCH v18 19/32] mm/swap.c: serialize memcg changes in pagevec_lru_move_fn Alex Shi
2020-09-22  0:42   ` Hugh Dickins
2020-09-22  5:00     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 20/32] mm/lru: replace pgdat lru_lock with lruvec lock Alex Shi
2020-09-22  5:27   ` Hugh Dickins
2020-09-22  8:58     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 21/32] mm/lru: introduce the relock_page_lruvec function Alex Shi
2020-09-22  5:40   ` Hugh Dickins
2020-08-24 12:54 ` [PATCH v18 22/32] mm/vmscan: use relock for move_pages_to_lru Alex Shi
2020-09-22  5:44   ` Hugh Dickins
2020-09-23  1:55     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 23/32] mm/lru: revise the comments of lru_lock Alex Shi
2020-09-22  5:48   ` Hugh Dickins
2020-08-24 12:54 ` [PATCH v18 24/32] mm/pgdat: remove pgdat lru_lock Alex Shi
2020-09-22  5:53   ` Hugh Dickins
2020-09-23  1:55     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 25/32] mm/mlock: remove lru_lock on TestClearPageMlocked in munlock_vma_page Alex Shi
2020-08-26  5:52   ` Alex Shi
2020-09-22  6:13   ` Hugh Dickins
2020-09-23  1:58     ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 26/32] mm/mlock: remove __munlock_isolate_lru_page Alex Shi
2020-08-24 12:55 ` [PATCH v18 27/32] mm/swap.c: optimizing __pagevec_lru_add lru_lock Alex Shi
2020-08-26  9:07   ` Alex Shi
2020-08-24 12:55 ` [PATCH v18 28/32] mm/compaction: Drop locked from isolate_migratepages_block Alex Shi
2020-08-24 12:55 ` [PATCH v18 29/32] mm: Identify compound pages sooner in isolate_migratepages_block Alex Shi
2020-08-24 12:55 ` [PATCH v18 30/32] mm: Drop use of test_and_set_skip in favor of just setting skip Alex Shi
2020-08-24 12:55 ` [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru_pages Alex Shi
2020-09-09  1:01   ` Matthew Wilcox
2020-09-09 15:43     ` Alexander Duyck
2020-09-09 17:07       ` Matthew Wilcox
2020-09-09 18:24       ` Hugh Dickins
2020-09-09 20:15         ` Matthew Wilcox
2020-09-09 21:05           ` Hugh Dickins
2020-09-09 21:17         ` Alexander Duyck
2020-08-24 12:55 ` [PATCH v18 32/32] mm: Split release_pages work into 3 passes Alex Shi
2020-08-24 18:42 ` [PATCH v18 00/32] per memcg lru_lock Andrew Morton
2020-08-24 20:24   ` Hugh Dickins
2020-08-25  1:56     ` Daniel Jordan
2020-08-25  3:26       ` Alex Shi
2020-08-25 11:39         ` Matthew Wilcox
2020-08-26  1:19         ` Daniel Jordan
2020-08-26  8:59           ` Alex Shi
2020-08-28  1:40             ` Daniel Jordan
2020-08-28  5:22               ` Alex Shi
2020-09-09  2:44               ` Aaron Lu
2020-09-09 11:40                 ` Michal Hocko
2020-08-25  8:52       ` Alex Shi
2020-08-25 13:00         ` Alex Shi
2020-08-27  7:01     ` Hugh Dickins
2020-08-27 12:20       ` Matthew Wilcox [this message]
2020-09-08 23:41       ` [PATCH v18 00/32] per memcg lru_lock: reviews Hugh Dickins
2020-09-09  2:24         ` Wei Yang
2020-09-09 15:08         ` Alex Shi
2020-09-09 23:16           ` Hugh Dickins
2020-09-11  2:50             ` Alex Shi
2020-09-12  2:13               ` Hugh Dickins
2020-09-13 14:21                 ` Alex Shi
2020-09-15  8:21                   ` Hugh Dickins
2020-09-15 16:58                     ` Daniel Jordan
2020-09-16 12:44                       ` Alex Shi
2020-09-17  2:37                       ` Alex Shi
2020-09-17 14:35                         ` Daniel Jordan
2020-09-17 15:39                           ` Alexander Duyck
2020-09-17 16:48                             ` Daniel Jordan
2020-09-12  8:38           ` Hugh Dickins
2020-09-13 14:22             ` Alex Shi
2020-09-09 16:11         ` Alexander Duyck
2020-09-10  0:32           ` Hugh Dickins
2020-09-10 14:24             ` Alexander Duyck
2020-09-12  5:12               ` Hugh Dickins
2020-08-25  7:21   ` [PATCH v18 00/32] per memcg lru_lock Michal Hocko

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