Re: [RFC, PATCH 6/6] ext3: do not write to the disk when mounting a dirty read-only filesystem

Jan Kara
2008-03-12T10:53:52Z

On Wed 12-03-08 02:42:46, Duane Griffin wrote:
> On Tue, Mar 11, 2008 at 04:11:23PM +0100, Jan Kara wrote:
> > Actually, this is nastier than it looks - currently fs, asks
> > ext3_sb_getblk() for block 'a' and it gets buffer head with b_blocknr == 'b'
> > instead.
>
> Note that it will be a different device as well, in the case of an
> external journal.
>
> > So when remounting you'd have to rewrite these buffers with
> > original block numbers which is not really possible. So I think
> > remapping will have to be solved differently, like providing buffer
> > head with correct b_blocknr but taking care when reading data to it and
> > reading them from elsewhere. Actually, this has to be done anyway
> > because JBD escapes data in the journal and you have to do unescaping
> > when reading data...
>
> Hmm, I'll think about this and try to get something working. As a quick
> proof-of-concept hack, getting both buffers then overwriting the fs
> block's data with the unescaped journal data should do the trick, right?
Yes, it should, but you should take care for users that do things like:

getblk(a)
ll_rw_block(READ, a)

or even

getblk(a)
submit_bh(a)

I'm not sure if there are any in ext3/ext4 but it definitely needs
checking.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR