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    Subject[20/90] ext4: Take page lock before looking at attached buffer_heads flags
    2.6.31-stable review patch.  If anyone has any objections, please let us know.

    ------------------
    (cherry picked from commit a827eaffff07c7d58a4cb32158cbeb4849f4e33a)

    In order to check whether the buffer_heads are mapped we need to hold
    page lock. Otherwise a reclaim can cleanup the attached buffer_heads.

    Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    ---
    fs/ext4/inode.c | 13 +++++++++++--
    1 file changed, 11 insertions(+), 2 deletions(-)

    --- a/fs/ext4/inode.c
    +++ b/fs/ext4/inode.c
    @@ -5298,12 +5298,21 @@ int ext4_page_mkwrite(struct vm_area_str
    else
    len = PAGE_CACHE_SIZE;

    + lock_page(page);
    + /*
    + * return if we have all the buffers mapped. This avoid
    + * the need to call write_begin/write_end which does a
    + * journal_start/journal_stop which can block and take
    + * long time
    + */
    if (page_has_buffers(page)) {
    - /* return if we have all the buffers mapped */
    if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
    - ext4_bh_unmapped))
    + ext4_bh_unmapped)) {
    + unlock_page(page);
    goto out_unlock;
    + }
    }
    + unlock_page(page);
    /*
    * OK, we need to fill the hole... Do write_begin write_end
    * to do block allocation/reservation.We are not holding



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