THP split makes non-atomic change of tail page flags. This is almost ok because tail pages are locked and isolated but this breaks recent changes in page locking: non-atomic operation could clear bit PG_waiters. As a result concurrent sequence get_page_unless_zero() -> lock_page() might block forever. Especially if this page was truncated later. Fix is trivial: clone flags before unfreezing page reference counter. This race exists since commit 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit") while unsave unfreeze itself was added in commit 8df651c7059e ("thp: cleanup split_huge_page()"). Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> --- mm/huge_memory.c | 25 +++++++++++++------------ 1 file changed, 13 insertions(+), 12 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 87ab9b8f56b5..2b38d9f2f262 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2357,6 +2357,19 @@ static void __split_huge_page_tail(struct page *head, int tail, VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail); VM_BUG_ON_PAGE(page_ref_count(page_tail) != 0, page_tail); + /* Clone page flags before unfreezing refcount. */ + page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP; + page_tail->flags |= (head->flags & + ((1L << PG_referenced) | + (1L << PG_swapbacked) | + (1L << PG_swapcache) | + (1L << PG_mlocked) | + (1L << PG_uptodate) | + (1L << PG_active) | + (1L << PG_locked) | + (1L << PG_unevictable) | + (1L << PG_dirty))); + /* * tail_page->_refcount is zero and not changing from under us. But * get_page_unless_zero() may be running from under us on the @@ -2375,18 +2388,6 @@ static void __split_huge_page_tail(struct page *head, int tail, page_ref_add(page_tail, 2); } - page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP; - page_tail->flags |= (head->flags & - ((1L << PG_referenced) | - (1L << PG_swapbacked) | - (1L << PG_swapcache) | - (1L << PG_mlocked) | - (1L << PG_uptodate) | - (1L << PG_active) | - (1L << PG_locked) | - (1L << PG_unevictable) | - (1L << PG_dirty))); - /* * After clearing PageTail the gup refcount can be released. * Page flags also must be visible before we make the page non-compound. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>