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    SubjectRe: [PATCH] reiserfs: don't drop PG_dirty when releasing sub-page-sized dirty file
    On Tue, Oct 23, 2007 at 12:07:07PM +0200, Peter Zijlstra wrote:
    > [ adding reiserfs devs to the CC ]

    Thank you.

    This fix is kind of crude - even when it fixed Maxim's problem, and
    survived my stress testing of a lot of patching and kernel compiling.
    I'd be glad to see better solutions.

    Fengguang
    ---

    reiserfs: don't drop PG_dirty when releasing sub-page-sized dirty file

    This is not a new problem in 2.6.23-git17.
    2.6.22/2.6.23 is buggy in the same way.

    Reiserfs could accumulate dirty sub-page-size files until umount time.
    They cannot be synced to disk by pdflush routines or explicit `sync'
    commands. Only `umount' can do the trick.

    The direct cause is: the dirty page's PG_dirty is wrongly _cleared_.
    Call trace:
    [<ffffffff8027e920>] cancel_dirty_page+0xd0/0xf0
    [<ffffffff8816d470>] :reiserfs:reiserfs_cut_from_item+0x660/0x710
    [<ffffffff8816d791>] :reiserfs:reiserfs_do_truncate+0x271/0x530
    [<ffffffff8815872d>] :reiserfs:reiserfs_truncate_file+0xfd/0x3b0
    [<ffffffff8815d3d0>] :reiserfs:reiserfs_file_release+0x1e0/0x340
    [<ffffffff802a187c>] __fput+0xcc/0x1b0
    [<ffffffff802a1ba6>] fput+0x16/0x20
    [<ffffffff8029e676>] filp_close+0x56/0x90
    [<ffffffff8029fe0d>] sys_close+0xad/0x110
    [<ffffffff8020c41e>] system_call+0x7e/0x83

    Fix the bug by removing the cancel_dirty_page() call. Tests show that
    it causes no bad behaviors on various write sizes.


    === for the patient ===
    Here are more detailed demonstrations of the problem.

    1) the page has both PG_dirty(D)/PAGECACHE_TAG_DIRTY(d) after being written to;
    and then only PAGECACHE_TAG_DIRTY(d) remains after the file is closed.

    ------------------------------ screen 0 ------------------------------
    [T0] root /home/wfg# cat > /test/tiny
    [T1] hi
    [T2] root /home/wfg#

    ------------------------------ screen 1 ------------------------------
    [T1] root /home/wfg# echo /test/tiny > /proc/filecache
    [T1] root /home/wfg# cat /proc/filecache
    # file /test/tiny
    # flags R:referenced A:active M:mmap U:uptodate D:dirty W:writeback O:owner B:buffer d:dirty w:writeback
    # idx len state refcnt
    0 1 ___UD__Bd_ 2
    [T2] root /home/wfg# cat /proc/filecache
    # file /test/tiny
    # flags R:referenced A:active M:mmap U:uptodate D:dirty W:writeback O:owner B:buffer d:dirty w:writeback
    # idx len state refcnt
    0 1 ___U___Bd_ 2

    2) note the non-zero 'cancelled_write_bytes' after /tmp/hi is copied.

    ------------------------------ screen 0 ------------------------------
    [T0] root /home/wfg# echo hi > /tmp/hi
    [T1] root /home/wfg# cp /tmp/hi /dev/stdin /test
    [T2] hi
    [T3] root /home/wfg#

    ------------------------------ screen 1 ------------------------------
    [T1] root /proc/4397# cd /proc/`pidof cp`
    [T1] root /proc/4713# cat io
    rchar: 8396
    wchar: 3
    syscr: 20
    syscw: 1
    read_bytes: 0
    write_bytes: 20480
    cancelled_write_bytes: 4096
    [T2] root /proc/4713# cat io
    rchar: 8399
    wchar: 6
    syscr: 21
    syscw: 2
    read_bytes: 0
    write_bytes: 24576
    cancelled_write_bytes: 4096

    //Question: the 'write_bytes' is a bit more than expected ;-)

    Cc: Maxim Levitsky <maximlevitsky@gmail.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Jeff Mahoney <jeffm@suse.com>
    Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
    ---
    fs/reiserfs/stree.c | 3 ---
    1 file changed, 3 deletions(-)

    --- linux-2.6.24-git17.orig/fs/reiserfs/stree.c
    +++ linux-2.6.24-git17/fs/reiserfs/stree.c
    @@ -1458,9 +1458,6 @@ static void unmap_buffers(struct page *p
    }
    bh = next;
    } while (bh != head);
    - if (PAGE_SIZE == bh->b_size) {
    - cancel_dirty_page(page, PAGE_CACHE_SIZE);
    - }
    }
    }
    }
    -
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