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    Subject[PATCH] mm: fix warning in __set_page_dirty_nobuffers
    New tmpfs use of !PageUptodate pages for fallocate() is triggering the
    WARNING: at mm/page-writeback.c:1990 when __set_page_dirty_nobuffers()
    is called from migrate_page_copy() for compaction.

    It is anomalous that migration should use __set_page_dirty_nobuffers()
    on an address_space that does not participate in dirty and writeback
    accounting; and this has also been observed to insert surprising dirty
    tags into a tmpfs radix_tree, despite tmpfs not using tags at all.

    We should probably give migrate_page_copy() a better way to preserve
    the tag and migrate accounting info, when mapping_cap_account_dirty().
    But that needs some more work: so in the interim, avoid the warning
    by using a simple SetPageDirty on PageSwapBacked pages.

    Reported-by: Dave Jones <davej@redhat.com>
    Signed-off-by: Hugh Dickins <hughd@google.com>
    ---

    mm/migrate.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- 3.4.0+/mm/migrate.c 2012-05-27 10:01:43.104049010 -0700
    +++ linux/mm/migrate.c 2012-06-01 00:10:58.080098749 -0700
    @@ -436,7 +436,10 @@ void migrate_page_copy(struct page *newp
    * is actually a signal that all of the page has become dirty.
    * Whereas only part of our page may be dirty.
    */
    - __set_page_dirty_nobuffers(newpage);
    + if (PageSwapBacked(page))
    + SetPageDirty(newpage);
    + else
    + __set_page_dirty_nobuffers(newpage);
    }

    mlock_migrate_page(newpage, page);

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