mm: try_to_unmap_cluster() should lock_page() before mlocking
A BUG_ON(!PageLocked) was triggered in mlock_vma_page() by Sasha Levin fuzzing with trinity. The call site try_to_unmap_cluster() does not lock the pages other than its check_page parameter (which is already locked). The BUG_ON in mlock_vma_page() is not documented and its purpose is somewhat unclear, but apparently it serializes against page migration, which could otherwise fail to transfer the PG_mlocked flag. This would not be fatal, as the page would be eventually encountered again, but NR_MLOCK accounting would become distorted nevertheless. This patch adds a comment to the BUG_ON in mlock_vma_page() and munlock_vma_page() to that effect. The call site try_to_unmap_cluster() is fixed so that for page != check_page, trylock_page() is attempted (to avoid possible deadlocks as we already have check_page locked) and mlock_vma_page() is performed only upon success. If the page lock cannot be obtained, the page is left without PG_mlocked, which is again not a problem in the whole unevictable memory design. Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Bob Liu <bob.liu@oracle.com> Reported-by: Sasha Levin <sasha.levin@oracle.com> Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com> Cc: Michel Lespinasse <walken@google.com> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Acked-by: Rik van Riel <riel@redhat.com> Cc: David Rientjes <rientjes@google.com> Cc: Mel Gorman <mgorman@suse.de> Cc: Hugh Dickins <hughd@google.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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2 changed files with 14 additions and 2 deletions
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@ -79,6 +79,7 @@ void clear_page_mlock(struct page *page)
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*/
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void mlock_vma_page(struct page *page)
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{
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/* Serialize with page migration */
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BUG_ON(!PageLocked(page));
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if (!TestSetPageMlocked(page)) {
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@ -174,6 +175,7 @@ unsigned int munlock_vma_page(struct page *page)
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unsigned int nr_pages;
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struct zone *zone = page_zone(page);
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/* For try_to_munlock() and to serialize with page migration */
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BUG_ON(!PageLocked(page));
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/*
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14
mm/rmap.c
14
mm/rmap.c
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@ -1332,9 +1332,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
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BUG_ON(!page || PageAnon(page));
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if (locked_vma) {
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mlock_vma_page(page); /* no-op if already mlocked */
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if (page == check_page)
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if (page == check_page) {
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/* we know we have check_page locked */
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mlock_vma_page(page);
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ret = SWAP_MLOCK;
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} else if (trylock_page(page)) {
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/*
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* If we can lock the page, perform mlock.
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* Otherwise leave the page alone, it will be
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* eventually encountered again later.
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*/
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mlock_vma_page(page);
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unlock_page(page);
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}
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continue; /* don't unmap */
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}
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