Hi Neil,
> Some devices - notably dm and md - can change their behaviour in
> response to BIO_RW_BARRIER requests. They might start out accepting
> such requests but on reconfiguration, they find out that they cannot
> any more.
>
> ext3 (and other filesystems) deal with this by always testing if
> BIO_RW_BARRIER requests fail with EOPNOTSUPP, and retrying the write
> requests without the barrier (probably after waiting for any pending
> writes to complete).
>
> However there is a bug in the handling for this for ext3.
>
> When ext3 (jbd actually) decides to submit a BIO_RW_BARRIER request,
> it sets the buffer_ordered flag on the buffer head.
> If the request completes successfully, the flag STAYS SET.
Yes, I've recently noted this as well :)
> Other code might then write the same buffer_head after the device has
> been reconfigured to not accept barriers. This write will then fail,
> but the "other code" is not ready to handle EOPNOTSUPP errors and the
> error will be treated as fatal.
>
> This can be seen without having to reconfigure a device at exactly the
> wrong time by putting:
>
> if (buffer_ordered(bh))
> printk("OH DEAR, and ordered buffer\n");
>
>
> in the while loop in "commit phase 5" of journal_commit_transaction.
>
> If it ever prints the "OH DEAR ..." message (as it does sometimes for
> me), then that request could (in different circumstances) have failed
> with EOPNOTSUPP, but that isn't tested for.
>
> My proposed fix is to clear the buffer_ordered flag after it has been
> used, as in the following patch.
Yes. Actually, I think there's another bug in there as well - at least
I have bugreport where we obviously miss that writing ordered buffer
failed (I see completion function of the buffer return with eopnotsupp
bit set but barriers aren't disabled). I think someone starts writing
out the buffer before we call sync_dirty_buffer(bh) but I'm not completely
sure who can do this when we just do set_buffer_dirty(bh)... Anyway
before I check that it is indeed happening what I think is happening,
your fix is fine :).
You can add: Acked-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Neil Brown <neilb@xxxxxxx>
>
> diff .prev/fs/jbd/commit.c ./fs/jbd/commit.c
> --- .prev/fs/jbd/commit.c 2008-02-07 10:01:57.000000000 +1100
> +++ ./fs/jbd/commit.c 2008-02-07 10:04:58.000000000 +1100
> @@ -131,6 +131,8 @@ static int journal_write_commit_record(j
> barrier_done = 1;
> }
> ret = sync_dirty_buffer(bh);
> + if (barrier_done)
> + clear_buffer_ordered(bh);
> /* is it possible for another commit to fail at roughly
> * the same time as this one? If so, we don't want to
> * trust the barrier flag in the super, but instead want
> @@ -148,7 +150,6 @@ static int journal_write_commit_record(j
> spin_unlock(&journal->j_state_lock);
>
> /* And try again, without the barrier */
> - clear_buffer_ordered(bh);
> set_buffer_uptodate(bh);
> set_buffer_dirty(bh);
> ret = sync_dirty_buffer(bh);
Honza
--
Jan Kara <jack@xxxxxxx>
SuSE CR Labs
-
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
