From 173d9d9fd3ddae84c110fea8aedf1f26af6be9ec Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Fri, 30 Nov 2018 14:10:16 -0800 Subject: [PATCH] mm/huge_memory: splitting set mapping+index before unfreeze Huge tmpfs stress testing has occasionally hit shmem_undo_range()'s VM_BUG_ON_PAGE(page_to_pgoff(page) != index, page). Move the setting of mapping and index up before the page_ref_unfreeze() in __split_huge_page_tail() to fix this: so that a page cache lookup cannot get a reference while the tail's mapping and index are unstable. In fact, might as well move them up before the smp_wmb(): I don't see an actual need for that, but if I'm missing something, this way round is safer than the other, and no less efficient. You might argue that VM_BUG_ON_PAGE(page_to_pgoff(page) != index, page) is misplaced, and should be left until after the trylock_page(); but left as is has not crashed since, and gives more stringent assurance. Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261516380.2275@eggly.anvils Fixes: e9b61f19858a5 ("thp: reintroduce split_huge_page()") Requires: 605ca5ede764 ("mm/huge_memory.c: reorder operations in __split_huge_page_tail()") Signed-off-by: Hugh Dickins Acked-by: Kirill A. Shutemov Cc: Konstantin Khlebnikov Cc: Jerome Glisse Cc: Matthew Wilcox Cc: [4.8+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/huge_memory.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 30100fac2341..cef2c256e7c4 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2402,6 +2402,12 @@ static void __split_huge_page_tail(struct page *head, int tail, (1L << PG_unevictable) | (1L << PG_dirty))); + /* ->mapping in first tail page is compound_mapcount */ + VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING, + page_tail); + page_tail->mapping = head->mapping; + page_tail->index = head->index + tail; + /* Page flags must be visible before we make the page non-compound. */ smp_wmb(); @@ -2422,12 +2428,6 @@ static void __split_huge_page_tail(struct page *head, int tail, if (page_is_idle(head)) set_page_idle(page_tail); - /* ->mapping in first tail page is compound_mapcount */ - VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING, - page_tail); - page_tail->mapping = head->mapping; - - page_tail->index = head->index + tail; page_cpupid_xchg_last(page_tail, page_cpupid_last(head)); /* -- 2.30.2