On Wed, 2008-02-06 at 22:25 -0600, Dave Kleikamp wrote:
> Duplicating Neil Brown's jbd patch for jbd2. I guess this can go
> through the ext4 queue rather than straight into -mm.
>
Checked-in.
Thanks Shaggy and Neil.
Mingming
> Neil's text:
>
> 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.
>
> 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.
>
> Thanks,
> NeilBrown
>
> Signed-off-by: Dave Kleikamp <shaggy@xxxxxxxxxxxxxxxxxx>
>
> diff -Nurp linux-2.6.24-mm1/fs/jbd2/commit.c linux/fs/jbd2/commit.c
> --- linux-2.6.24-mm1/fs/jbd2/commit.c 2008-02-04 09:08:44.000000000 -0600
> +++ linux/fs/jbd2/commit.c 2008-02-06 22:11:14.000000000 -0600
> @@ -148,6 +148,8 @@ static int journal_submit_commit_record(
> barrier_done = 1;
> }
> ret = submit_bh(WRITE, 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
> @@ -166,7 +168,6 @@ static int journal_submit_commit_record(
> 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 = submit_bh(WRITE, bh);
>
>
> -
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