On Thu 05-08-10 12:09:41, Andrew Morton wrote:
> On Thu, 5 Aug 2010 18:42:16 +0200
> Jan Kara <jack@xxxxxxx> wrote:
>
> > In data=journal mode, we still use block_write_begin() to prepare page for
> > writing. This function can occasionally mark buffer dirty which violates
> > journalling assumptions - when a buffer is part of a transaction, it should be
> > dirty and a buffer can be already part of a forget list of some transaction
> > when block_write_begin() gets called. This violation of journalling assumptions
> > then results in "JBD: Spotted dirty metadata buffer..." warnings.
> >
> > In fact, temporary dirtying the buffer while the page is still locked does not
> > really cause problems to the journalling because we won't write the buffer
> > until the page gets unlocked. So we just have to make sure to clear dirty bits
> > before unlocking the page.
> >
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > ---
> > fs/ext3/inode.c | 18 +++++++++++++++++-
> > 1 files changed, 17 insertions(+), 1 deletions(-)
> >
> > diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
> > index 735f019..1a84abb 100644
> > --- a/fs/ext3/inode.c
> > +++ b/fs/ext3/inode.c
> > @@ -1149,9 +1149,25 @@ static int walk_page_buffers( handle_t *handle,
> > static int do_journal_get_write_access(handle_t *handle,
> > struct buffer_head *bh)
> > {
> > + int dirty = buffer_dirty(bh);
> > + int ret;
> > +
> > if (!buffer_mapped(bh) || buffer_freed(bh))
> > return 0;
> > - return ext3_journal_get_write_access(handle, bh);
> > + /*
> > + * __block_prepare_write() could have dirtied some buffers. Clean
> > + * the dirty bit as jbd2_journal_get_write_access() could complain
> > + * otherwise about fs integrity issues. Setting of the dirty bit
> > + * by __block_prepare_write() isn't a real problem here as we clear
> > + * the bit before releasing a page lock and thus writeback cannot
> > + * ever write the buffer.
> > + */
> > + if (dirty)
> > + clear_buffer_dirty(bh);
>
> mark_buffer_dirty() can run set_page_dirty() which will set the page
> dirty and increment dirty-page accounting. If we then run
> clear_buffer_dirty() we can end up with a dirty page which has clean
> buffers and an off-by-one in dirty-page accounting.
>
> Later, writeback will come along and will attempt to write the "dirty"
> page. It will discover the cleanness of the buffers, will mark the
> page clean without doing any IO and will decrement the dirty-page
> accounting. So everything gets fixed up again.
>
> So I don't see any problem here and this isn't the only place where
> this sort of thing occurs. It's just somethnig to be aware of and to
> have a think about.
Yes, JBD tends to do this a lot (with all the journaled buffers).
Thanks for review!
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
