Re: [PATCH 2/2] ext4: journal superblock modifications in ext4_statfs()

[Duane Griffin <duaneg@dghda.com>]

2009/11/19  <tytso@mit.edu>:
> On Mon, Nov 16, 2009 at 03:38:16PM -0800, Andreas Dilger wrote:
>> The other thing that comes to mind is that we don't recover the journal
>> for a read-only e2fsck, but we DO recover it on a read-only mount
>> seems inconsistent.  It wouldn't be hard to have e2fsck -n read the
>> journal and
>> persistently cache the journal blocks in its internal cache (i.e. flag
>> them so they can't be discarded from cache) before it runs the rest
>> of the
>> e2fsck.
>
> Eventually it would be nice if we did the same thing in both kernel
> and userspace when doing a read-only mount/check: build a redirection
> table that maps specific physical blocks to the block in the journal,
> and whenever the system tries to access a specific physical block, we
> look up the proper block to use instead in the redirection block.

Unfortunately you can't just blindly give back the journalled block:
it may have been escaped. So you need to read in the block from the
journal, unescape it if required, then give it back.

> The one tricky bit about doing this in the kernel is that we would
> still have to replay the journal in the case of the read-only root.
> Why?  Because otherwise older e2fsck's would get confused and replay
> the journal, and that would lead to some potentially serious
> confusion.  Even if we fix this in future versions of e2fsck, we still
> need to be careful dealing with remounting a r/o filesystem to be
> read/write, especially in the journal=data mode.

Hmm. The e2fsck confusion is an interesting wrinkle.

> The simple way of handling journaled data blocks is to hack the
> bmap() function to use the redirection block, but the problem with
> doing that is the journal block will be left in the buffer heads in
> the page cache.  If the file system is remounted r/w without first
> flushing these buffer heads, future attempts to modify these pages in
> the page cache could result in a random block in the journalling
> getting corrupted by an update, instead of updating the proper final
> location on disk for that data block.

Yes, they certainly need to be flushed.

> If we have someone who is at least some basic experience in kernel
> coding, but and an entry-level project getting involved with ext4,
> this would be an ideal, self-contained thing to try doing.  I'd
> suggest implementing it in userspace first, using the userspace/kernel
> API framework that allows e2fsck/recovery.c to be roughly kept in sync
> with fs/jbd[2]/recovery.c, and avoiding the hair of r/o roots by
> always replaying the journal in the case of the root file system.
> Anyone interested?  If so, let me know...

I am (still) interested in this. I'll have a look at the userspace
side of things.

>                                                       - Ted

Cheers,
Duane.

-- 
"I never could learn to drink that blood and call it wine" - Bob Dylan