Date:	Wed, 13 Aug 2014 15:53:58 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	TR Reardon <thomas_reardon@...mail.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: journal_checksum_v2 on-disk format change? (was: Re: journal
 recovery problems with metadata_csum, *non-64bit*)

On Wed, Aug 13, 2014 at 11:35:50PM +0200, Andreas Dilger wrote:
> Doesn't a larger journal_tag_bytes() size mean more overhead for
> the journal?

Yes.  If we move to a 16-byte block tag:

typedef struct journal_block_tag3_s
{
	__u32		t_blocknr;	/* The on-disk block number */
	__u32		t_flags;	/* See below */
	__u32		t_blocknr_high; /* most-significant high 32bits. */
	__u32		t_checksum;	/* crc32c(uuid+seq+block) */
} journal_block_tag3_t;

Notice that this fixes the alignment problem due to the v2 bug -- tag 2 starts
with a 4-byte field at offset 14.

For a 32-bit FS with 4K blocks and a 32768 block journal, I found through
experimentation that if I fill the journal with nothing but journal blocks
(i.e. no revoke blocks), overhead increases by 264 blocks, or 0.8%.  For a
64-bit FS with the same parameters, the increase is 136 blocks, or 0.4%.

I'm hoping that a <1% increase won't discourage people. :)

--D
> 
> Cheers, Andreas
> 
> > On Aug 12, 2014, at 19:08, "Darrick J. Wong" <darrick.wong@...cle.com> wrote:
> > 
> >> On Tue, Aug 12, 2014 at 01:39:35AM -0400, TR Reardon wrote:
> >>> On 8/11/14, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> >>> Hi all,
> >>> 
> >>> Mr. Reardon has discovered that due to a bug in journal_tag_bytes(), if the
> >>> the
> >>> "journal csum v2" feature flag is turned on, block tag records are being
> >>> written with two extra bytes of space because we don't need to execute
> >>> "x += sizeof(tag.t_checksum);".  In 32-bit mode, other parts of the journal
> >>> then perform incorrect size comparisons, leading to BUG() being called.  In
> >>> 64-bit mode, there's enough padding that bad things won't happen.
> >>> 
> >>> This is a remnant of the days when I tried to enlarge journal_block_tag_t
> >>> to
> >>> hold the full 32-bit checksum for a data block that's stored in the
> >>> journal.
> >>> Back in 2011, we decided (though sadly I can't find the link; I think we
> >>> might
> >>> have discussed this in the concall) that it was better not to change the
> >>> size
> >>> of journal_block_tag_t than it was to make it bigger so that it could hold
> >>> the
> >>> full checksum.
> >>> 
> >>> A simple fix for the problem has been proposed by Mr. Reardon which fixes
> >>> journal_tag_bytes() and leaves everything else unchanged.  However, that is
> >>> technically a disk format change since the size of journal_block_tag_t on
> >>> disk
> >>> changes, albeit only for people running experimental metadata_csum
> >>> filesystems.
> >>> Since we've been allocating disk space for the enlarged checksum this whole
> >>> time, if we apply that patch, anyone with an unclean 64bit FS will not be
> >>> able
> >>> to recover the journal.  (Anyone with an unclean 32-bit FS has been broken
> >>> the
> >>> whole time, and still will be.)
> >>> 
> >>> The other thing we could do is actually use the two bytes to store the high
> >>> 16-bits of the checksum, fix the jbd2 helper functions to reflect that
> >>> reality
> >>> (so that they don't BUG()), and change the checksum verify routine to pass
> >>> the
> >>> block if either the full checksum matches, or if the lower 16 bits match
> >>> and
> >>> the upper 16 bits are zero.  With this route, anybody with an uncleanly
> >>> unmounted FS could still recover the journal, since we're not changing the
> >>> size
> >>> of anything.
> >>> 
> >>> Fortunately, journal tag blocks are fairly ephemeral and nobody ought to be
> >>> using metadata_csum on a production filesystem, so at this point we can
> >>> probably change the disk format without too many repercussions.  If you
> >>> make
> >>> sure to cleanly unmount the filesystem when changing kernel/e2fsprogs
> >>> versions,
> >>> there will be no problems.
> >>> 
> >>> So, uh... comments?  How should we proceed?
> >>> 
> >>> --D
> >> 
> >> My only concern is that legacy applies to in-the-wild kernels, not
> >> just journals.  Any post 3.4 kernel has this problem, which will be
> >> exposed as soon as e2fsprogs 1.43 is released.  A common (enough) use
> >> case might be, say, a 2TB USB drive, being unplugged and replugged
> >> across machines  without proper shutdown.  Old machines will think
> >> they recognize the journal but don't actually, and replay something
> >> destructive.
> >> 
> >> In other words, this is a future-retro problem.  The problem is not so
> >> much with existing fs experimenting with metadata_csum, but a future
> >> properly-journaled drive being plugged into an legacy faulty machine.
> >> Those incompat flags are supposed to protect against just this
> >> scenario, but won't.  So perhaps you'll need make a new
> >> "INCOMPAT_CSUM_V3" flag to protect?  Actually, dwelling on this a bit
> >> today, I think whether or not you retain those 2 extra checksum bytes
> >> in final fix, you ought use a new flag for the format.
> > 
> > Definitely we need a new bit to keep the old kernels out; I wanted others to
> > weigh in on whether we ought to shrink the struct or put the two bytes to use.
> > 
> > That said... journal recovery is not so well tested, so I'm currently busy
> > building out debugfs to create journal transactions so we can test recovery
> > both with and without *_csum.
> > 
> > --D
> >> 
> >> +Reardon
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