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    SubjectRe: data corrupting bug in 2.4.20 ext3, data=journal
    On Mon, 2002-12-02 at 12:15, Stephen C. Tweedie wrote:

    > The problem is that ext3 is expecting that truncate_inode_pages() (and
    > hence ext3_flushpage) is only called during a truncate. That's what the
    > function is named for, after all, and it's the hint we need to indicate
    > that future writeback on the data we're discarding should be disabled
    > (so that we don't get old data written on top of new data should the
    > block get deallocated.)
    > But kill_supers() eventually calls truncate_inode_pages() too when we're
    > doing the invalidate_inodes().

    But we've already called fsync_super() at this point. If that completes
    synchronously, then the buffers will already be out of the journal and
    queued for writeback when we get here, and clearing BH_JBDDirty won't
    have any dire consequences.

    Indeed, loading the ext3 module with "do_sync_supers=1" cures the

    However, doing every superblock write synchronously is a non-starter, as
    that requires a journal commit in the ext3 universe, and so this would
    essentially force bdflush to serialise all of its filesystem flushes
    (which is a real problem once you've got a significant number of
    filesystems mounted.) But the VFS simply doesn't have any clean way of
    telling foo_write_super() whether the call needs to be completed
    synchronously or asynchronously.

    There *is* a totally unclean way, though. kill_super() sets sb->s_root
    to NULL before calling its final fsync_super(), and ext3 can easily
    check that in ext3_write_super(). It's a nasty, messy dependency, but
    should work for 2.4 at least. For 2.5 we probably want to look at
    passing an async flag down into the write_super.

    The attached patch seems to fix things for me.


    --- linux-2.4-ext3merge/fs/ext3/super.c.=K0027=.orig 2002-12-02 15:35:13.000000000 +0000
    +++ linux-2.4-ext3merge/fs/ext3/super.c 2002-12-02 15:35:14.000000000 +0000
    @@ -1640,7 +1640,12 @@
    sb->s_dirt = 0;
    target = log_start_commit(EXT3_SB(sb)->s_journal, NULL);

    - if (do_sync_supers) {
    + /*
    + * Tricky --- if we are unmounting, the write really does need
    + * to be synchronous. We can detect that by looking for NULL in
    + * sb->s_root.
    + */
    + if (do_sync_supers || !sb->s_root) {
    log_wait_commit(EXT3_SB(sb)->s_journal, target);
     \ /
      Last update: 2005-03-22 13:31    [W:0.020 / U:17.492 seconds]
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