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    Subject[RFC][PATCH] mm: write_cache_pages be more sequential
    From 6f3bb7c26936c45d810048f59c369e8d5a5623fc Mon Sep 17 00:00:00 2001
    From: Adrian Hunter <adrian.hunter@nokia.com>
    Date: Mon, 7 Sep 2009 10:49:11 +0300
    Subject: [PATCH] mm: write_cache_pages be more sequential

    If a file is written to sequentially, then writeback
    should write the pages sequentially also. However,
    that does not always happen. For example:

    1) user writes pages 0, 1 and 2 but 2 is incomplete
    2) write_cache_pages writes pages 0, 1 and 2 and sets
    writeback_index to 3
    3) user finishes writing page 2 and writes pages 3 and 4
    4) write_cache_pages writes pages 3 and 4, and then cycles
    back and writes page 2 again.

    So the pages are written out in the order 0, 1, 2, 3 ,4 ,2
    instead of 0, 1, 2, 2, 3, 4.

    This situation was noticed on UBIFS because it writes
    directly from writepage. Hence if there is an unexpected
    power-loss, a file will end up with a hole even though
    the file was written sequentially by the user.

    Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
    ---
    mm/page-writeback.c | 2 ++
    1 files changed, 2 insertions(+), 0 deletions(-)

    diff --git a/mm/page-writeback.c b/mm/page-writeback.c
    index 81627eb..7410b7a 100644
    --- a/mm/page-writeback.c
    +++ b/mm/page-writeback.c
    @@ -960,6 +960,8 @@ int write_cache_pages(struct address_space *mapping,
    pagevec_init(&pvec, 0);
    if (wbc->range_cyclic) {
    writeback_index = mapping->writeback_index; /* prev offset */
    + if (writeback_index)
    + writeback_index -= 1;
    index = writeback_index;
    if (index == 0)
    cycled = 1;
    --
    1.5.6.3

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