linux-ext4 - Re: ext4 fsck vs. kernel recovery policy

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Date:   Wed, 4 Sep 2019 08:58:57 -0600
From:   dann frazier <dann.frazier@...onical.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Andreas Dilger <adilger@...ger.ca>, linux-ext4@...r.kernel.org,
        Jan Kara <jack@...e.com>,
        Colin King <colin.king@...onical.com>,
        Ryan Harper <ryan.harper@...onical.com>
Subject: Re: ext4 fsck vs. kernel recovery policy

On Thu, Aug 29, 2019 at 09:22:36PM -0400, Theodore Y. Ts'o wrote:
> (Changing the cc from linux-fsdevel to linux-ext4.)
> 
> On Thu, Aug 29, 2019 at 04:53:48PM -0600, dann frazier wrote:
> > JBD2: Invalid checksum recovering data block 517634 in log
> > 
> > So is it correct to say that the checksum errors were identifying
> > filesystem correctness issues, and therefore e2fsck was needed to
> > correct them?
> 
> That's correct.  More precisely, checksum errors for journal blocks
> are presumed to mean that file system might be corrupt, so a full
> e2fsck check was needed to make sure the file system was consistent.
> 
> > You're probably right - this issue is very easy to reproduce w/
> > data=journal,journal_checksum. I was never able to reproduce it
> > otherwise.
> 
> I've looked at the data block numbers that you've reported, and they
> come from a journald file.  The problem is with data=journal +
> journal_checksum + mmap.  Unfortunately, we don't handle that
> combination correctly at the moment.
> 
> The fix is going to have to involve fixing __ext4_journalled_writepage()
> to call set_page_writeback() before it unlocks the page, adding a list of
> pages under data=journalled writeback which is attached to the
> transaction handle, have the jbd2 commit hook call end_page_writeback()
> on all of these pages, and then in the places where ext4 calls
> wait_for_stable_page() or grab_cache_page_write_begin(),
> we need to add:
> 
> 	if (ext4_should_journal_data(inode))
> 		wait_on_page_writeback(page);
> 
> It's all relatively straightforward except for the part where we have to
> attach a list of pages to the currently running transaction.  That
> will require adding  some plumbing into the jbd2 layer.
> 
> Dann, any interest in trying to code this fix?

Thanks Ted. I've the interest, I'll see if I can find the time :)

  -dann