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    Subject[PATCH 03/35] writeback: prevent duplicate balance_dirty_pages_ratelimited() calls
    When dd in 512bytes, balance_dirty_pages_ratelimited() used to be called
    8 times for the same page, even if the page is only dirtied once. Fix it
    with a (slightly racy) PageDirty() test.

    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/filemap.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- linux-next.orig/mm/filemap.c 2010-12-13 21:45:57.000000000 +0800
    +++ linux-next/mm/filemap.c 2010-12-13 21:46:11.000000000 +0800
    @@ -2244,6 +2244,7 @@ static ssize_t generic_perform_write(str
    long status = 0;
    ssize_t written = 0;
    unsigned int flags = 0;
    + unsigned int dirty;

    /*
    * Copies from kernel address space cannot fail (NFSD is a big user).
    @@ -2292,6 +2293,7 @@ again:
    pagefault_enable();
    flush_dcache_page(page);

    + dirty = PageDirty(page);
    mark_page_accessed(page);
    status = a_ops->write_end(file, mapping, pos, bytes, copied,
    page, fsdata);
    @@ -2318,7 +2320,8 @@ again:
    pos += copied;
    written += copied;

    - balance_dirty_pages_ratelimited(mapping);
    + if (!dirty)
    + balance_dirty_pages_ratelimited(mapping);

    } while (iov_iter_count(i));




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