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    SubjectRe: [PATCH 3/4] writeback: pay attention to wbc->nr_to_write in write_cache_pages
    On Tue, 20 Apr 2010 12:41:53 +1000
    Dave Chinner <david@fromorbit.com> wrote:

    > If a filesystem writes more than one page in ->writepage, write_cache_pages
    > fails to notice this and continues to attempt writeback when wbc->nr_to_write
    > has gone negative - this trace was captured from XFS:
    >
    >
    > wbc_writeback_start: towrt=1024
    > wbc_writepage: towrt=1024
    > wbc_writepage: towrt=0
    > wbc_writepage: towrt=-1
    > wbc_writepage: towrt=-5
    > wbc_writepage: towrt=-21
    > wbc_writepage: towrt=-85
    >

    Bug.

    AFAIT it's a regression introduced by

    : commit 17bc6c30cf6bfffd816bdc53682dd46fc34a2cf4
    : Author: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    : AuthorDate: Thu Oct 16 10:09:17 2008 -0400
    : Commit: Theodore Ts'o <tytso@mit.edu>
    : CommitDate: Thu Oct 16 10:09:17 2008 -0400
    :
    : vfs: Add no_nrwrite_index_update writeback control flag

    I suggest that what you do here is remove the local `nr_to_write' from
    write_cache_pages() and go back to directly using wbc->nr_to_write
    within the loop.

    And thus we restore the convention that if the fs writes back more than
    a single page, it subtracts (nr_written - 1) from wbc->nr_to_write.



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