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Message-ID: <20120430155341.GC6938@tux1.beaverton.ibm.com>
Date: Mon, 30 Apr 2012 08:53:41 -0700
From: djwong <djwong@...ibm.com>
To: "Ted Ts'o" <tytso@....edu>,
Andreas Dilger <adilger.kernel@...ger.ca>,
Sunil Mushran <sunil.mushran@...cle.com>,
Martin K Petersen <martin.petersen@...cle.com>,
Greg Freemyer <greg.freemyer@...il.com>,
Amir Goldstein <amir73il@...il.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Andi Kleen <andi@...stfloor.org>,
Mingming Cao <cmm@...ibm.com>,
Joel Becker <jlbec@...lplan.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
linux-ext4@...r.kernel.org, Coly Li <colyli@...il.com>
Subject: Re: [PATCH 15/23] jbd2: Change disk layout for metadata
checksumming
On Sat, Apr 28, 2012 at 10:19:33AM -0400, Ted Ts'o wrote:
> On Tue, Mar 06, 2012 at 12:49:41PM -0800, Darrick J. Wong wrote:
> > @@ -177,11 +189,17 @@ typedef struct journal_block_tag_s
> > __be32 t_blocknr; /* The on-disk block number */
> > __be32 t_flags; /* See below */
> > __be32 t_blocknr_high; /* most-significant high 32bits. */
> > + __be32 t_checksum; /* crc32c(uuid+seq+block) */
> > } journal_block_tag_t;
> >
> > #define JBD2_TAG_SIZE32 (offsetof(journal_block_tag_t, t_blocknr_high))
> > #define JBD2_TAG_SIZE64 (sizeof(journal_block_tag_t))
>
> There's a problem with this patch here --- we are changing the size of
> journal_block_tag_t, which is an on-disk data structure. So for
> 64-bit journals, this represents a format change. This means that if
> you have a 64-bit file system that needs to have its journal
> recovered, if the journal was written with an older kernel, and then
> we try to recover it with a new kernel, things won't be good.
> Similarly, for e2fsck's recovery code, it's not going to be able to
> recover 64-bit file systems using current coding, since this patch
> series changes the size of JBD2_TAG_SIZE64.
>
> What we need to do is something like this:
>
> #define JBD2_TAG_SIZE64 (offsetof(journal_block_tag_t, t_checksum))
> #define JBD2_TAG_SIZE_CSUM (sizeof(journal_block_tag_t))
>
> And then change the code appropriately in e2fsprogs and in the kernel
> to use the correct tag size depending on the journal options.
Oops. I forgot to update JBD2_TAG_SIZE64.
I have a question, though -- it looks as though the code that handles reading
and writing tags from raw disk blocks calls journal_tag_bytes() to determine
the tag size, and manually increments a pointer "tagp" to step through the
block. This construction seems to be be sufficient to deal with possible
differences between sizeof(journal_block_tag_t) and the on-disk tag size, and
both increases over the 32bit tag size are gated on INCOMPAT_64BIT and
INCOMPAT_CSUM_V2.
Had I defined JBD2_TAG_SIZE64 with offsetof() as Ted did above, I think that
journal_tag_bytes() would return the correct on-disk tag size, which should fix
the scenario Ted outlined above. The tag checksum set/verify functions would
also need to be taught where t_checksum is (in the space occupied by
t_blocknr_high) on a 32bit journal. Could those two suggestions fix the
problem without causing us to discard half the checksum bits?
Well, not quite -- the calculation of tags per block in journal.c below the
comment "journal descriptor can store up to n blocks -bzzz" probably ought to
be using journal_tag_bytes(), not sizeof(journal_block_tag_t) to figure out how
many tags can be crammed into a disk block, since right now I think it
underreports the number of tags per block on a 32bit journal.
journal_tag_disk_size() is a more descriptive name for journal_tag_bytes().
As for putting half the checksum into the upper 16 bits of the flags field --
is journal space at such a premium that we need to overload the field and
reduce the strength of the checksum? Enabling journal checksums on a 4k block
filesystem causes tags_per_block to decrease from 512 to 341 on a 32bit journal
and from 341 to 256 on a 64bit journal. Do transactions typically have that
many blocks? I didn't think most transactions had 1-2MB of dirty data.
--D
>
> - Ted
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