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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 30100fa..cef2c25 100644 (file)
--- 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));
 
        /*