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Message-Id: <FCF93AD5-A078-4855-BC8D-B7B046884411@dilger.ca> Date: Sat, 28 Apr 2012 16:58:12 -0600 From: Andreas Dilger <adilger@...ger.ca> To: "Ted Ts'o" <tytso@....edu> Cc: "Darrick J. Wong" <djwong@...ibm.com>, 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" <linux-ext4@...r.kernel.org>, Coly Li <colyli@...il.com> Subject: Re: [PATCH 15/23] jbd2: Change disk layout for metadata checksumming On 2012-04-28, at 8:19, Ted Ts'o <tytso@....edu> 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. I thought we originally discussed using the high 16 bits of the t_flags field to store the checksum? This would avoid the need to change the disk format. Since there is still a whole transaction checksum, it isn't so critical that the per-block checksum be strong. One idea is to do the crc32c for each block, then store the high 16 bits into t_flags, and checksum the full 32-bit per-block checksums to make the commit block checksum, to avoid having to do the block checksums twice. Cheers, Andreas-- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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