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

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Message-ID: <20190830012236.GC10779@mit.edu>
Date:   Thu, 29 Aug 2019 21:22:36 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     dann frazier <dann.frazier@...onical.com>
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

(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?

      	  	      	     	     	  - Ted