From: Vlastimil Babka <[email protected]> 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. Reported-by: Sasha Levin <[email protected]> Cc: Wanpeng Li <[email protected]> Cc: Michel Lespinasse <[email protected]> Cc: KOSAKI Motohiro <[email protected]> Cc: Rik van Riel <[email protected]> Cc: David Rientjes <[email protected]> Cc: Mel Gorman <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: Joonsoo Kim <[email protected]> Cc: <[email protected]> Signed-off-by: Vlastimil Babka <[email protected]> Signed-off-by: Bob Liu <[email protected]> --- mm/mlock.c | 2 ++ mm/rmap.c | 14 ++++++++++++-- 2 files changed, 14 insertions(+), 2 deletions(-) diff --git a/mm/mlock.c b/mm/mlock.c index 4e1a6816..b1eb536 100644 --- a/mm/mlock.c +++ b/mm/mlock.c @@ -79,6 +79,7 @@ void clear_page_mlock(struct page *page) */ void mlock_vma_page(struct page *page) { + /* Serialize with page migration */ BUG_ON(!PageLocked(page)); if (!TestSetPageMlocked(page)) { @@ -174,6 +175,7 @@ unsigned int munlock_vma_page(struct page *page) unsigned int nr_pages; struct zone *zone = page_zone(page); + /* For try_to_munlock() and to serialize with page migration */ BUG_ON(!PageLocked(page)); /* diff --git a/mm/rmap.c b/mm/rmap.c index d9d4231..43d429b 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1322,9 +1322,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount, BUG_ON(!page || PageAnon(page)); if (locked_vma) { - mlock_vma_page(page); /* no-op if already mlocked */ - if (page == check_page) + if (page == check_page) { + /* we know we have check_page locked */ + mlock_vma_page(page); ret = SWAP_MLOCK; + } else if (trylock_page(page)) { + /* + * If we can lock the page, perform mlock. + * Otherwise leave the page alone, it will be + * eventually encountered again later. + */ + mlock_vma_page(page); + unlock_page(page); + } continue; /* don't unmap */ } -- 1.7.10.4 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
