From: Hugh Dickins hughd@google.com Subject: 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 hughd@google.com Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Konstantin Khlebnikov khlebnikov@yandex-team.ru Cc: Jerome Glisse jglisse@redhat.com Cc: Matthew Wilcox willy@infradead.org Cc: stable@vger.kernel.org [4.8+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/huge_memory.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/mm/huge_memory.c~mm-huge_memory-splitting-set-mappingindex-before-unfreeze +++ a/mm/huge_memory.c @@ -2402,6 +2402,12 @@ static void __split_huge_page_tail(struc (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(struc 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));

/* _