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    Subject[PATCH 4/5] writeback: remove pages_skipped accounting in __block_write_full_page()
    Miklos Szeredi <miklos@szeredi.hu> and me identified a writeback bug:

    > The following strange behavior can be observed:
    >
    > 1. large file is written
    > 2. after 30 seconds, nr_dirty goes down by 1024
    > 3. then for some time (< 30 sec) nothing happens (disk idle)
    > 4. then nr_dirty again goes down by 1024
    > 5. repeat from 3. until whole file is written
    >
    > So basically a 4Mbyte chunk of the file is written every 30 seconds.
    > I'm quite sure this is not the intended behavior.

    It can be produced by the following test scheme:

    # cat bin/test-writeback.sh
    grep nr_dirty /proc/vmstat
    echo 1 > /proc/sys/fs/inode_debug
    dd if=/dev/zero of=/var/x bs=1K count=204800&
    while true; do grep nr_dirty /proc/vmstat; sleep 1; done

    # bin/test-writeback.sh
    nr_dirty 19207
    nr_dirty 19207
    nr_dirty 30924
    204800+0 records in
    204800+0 records out
    209715200 bytes (210 MB) copied, 1.58363 seconds, 132 MB/s
    nr_dirty 47150
    nr_dirty 47141
    nr_dirty 47142
    nr_dirty 47142
    nr_dirty 47142
    nr_dirty 47142
    nr_dirty 47205
    nr_dirty 47214
    nr_dirty 47214
    nr_dirty 47214
    nr_dirty 47214
    nr_dirty 47214
    nr_dirty 47215
    nr_dirty 47216
    nr_dirty 47216
    nr_dirty 47216
    nr_dirty 47154
    nr_dirty 47143
    nr_dirty 47143
    nr_dirty 47143
    nr_dirty 47143
    nr_dirty 47143
    nr_dirty 47142
    nr_dirty 47142
    nr_dirty 47142
    nr_dirty 47142
    nr_dirty 47134
    nr_dirty 47134
    nr_dirty 47135
    nr_dirty 47135
    nr_dirty 47135
    nr_dirty 46097 <== -1038
    nr_dirty 46098
    nr_dirty 46098
    nr_dirty 46098
    [...]
    nr_dirty 46091
    nr_dirty 46092
    nr_dirty 46092
    nr_dirty 45069 <== -1023
    nr_dirty 45056
    nr_dirty 45056
    nr_dirty 45056
    [...]
    nr_dirty 37822
    nr_dirty 36799 <== -1023
    [...]
    nr_dirty 36781
    nr_dirty 35758 <== -1023
    [...]
    nr_dirty 34708
    nr_dirty 33672 <== -1024
    [...]
    nr_dirty 33692
    nr_dirty 32669 <== -1023


    % ls -li /var/x
    847824 -rw-r--r-- 1 root root 200M 2007-08-12 04:12 /var/x

    % dmesg|grep 847824 # generated by a debug printk
    [ 529.263184] redirtied inode 847824 line 548
    [ 564.250872] redirtied inode 847824 line 548
    [ 594.272797] redirtied inode 847824 line 548
    [ 629.231330] redirtied inode 847824 line 548
    [ 659.224674] redirtied inode 847824 line 548
    [ 689.219890] redirtied inode 847824 line 548
    [ 724.226655] redirtied inode 847824 line 548
    [ 759.198568] redirtied inode 847824 line 548

    # line 548 in fs/fs-writeback.c:
    543 if (wbc->pages_skipped != pages_skipped) {
    544 /*
    545 * writeback is not making progress due to locked
    546 * buffers. Skip this inode for now.
    547 */
    548 redirty_tail(inode);
    549 }

    More debug efforts show that __block_write_full_page()
    never has the chance to call submit_bh() for that big dirty file:
    the buffer head is *clean*. So basicly no page io is issued by
    __block_write_full_page(), hence pages_skipped goes up.

    Also the comment in generic_sync_sb_inodes():

    544 /*
    545 * writeback is not making progress due to locked
    546 * buffers. Skip this inode for now.
    547 */

    and the comment in __block_write_full_page():

    1713 /*
    1714 * The page was marked dirty, but the buffers were
    1715 * clean. Someone wrote them back by hand with
    1716 * ll_rw_block/submit_bh. A rare case.
    1717 */

    do not quite agree with each other. The page writeback should be skipped for
    'locked buffer', but here it is 'clean buffer'!

    This patch fixes this bug. Though I'm not sure why __block_write_full_page()
    is called only to do nothing and who actually issued the writeback for us.

    This is the two possible new behaviors after the patch:

    1) pretty nice: wait 30s and write ALL:)
    2) not so good:
    - during the dd: ~16M
    - after 30s: ~4M
    - after 5s: ~4M
    - after 5s: ~176M

    The next patch will fix case (2).

    Cc: David Chinner <dgc@sgi.com>
    Cc: Ken Chen <kenchen@google.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
    ---
    fs/buffer.c | 1 -
    1 file changed, 1 deletion(-)

    --- linux-2.6.23-rc8-mm2.orig/fs/buffer.c
    +++ linux-2.6.23-rc8-mm2/fs/buffer.c
    @@ -1737,7 +1737,6 @@ done:
    * The page and buffer_heads can be released at any time from
    * here on.
    */
    - wbc->pages_skipped++; /* We didn't write this page */
    }
    return err;

    --
    -
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