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Date: Mon, 5 Dec 2011 11:45:39 -0800 From: "Darrick J. Wong" <djwong@...ibm.com> To: "Ted Ts'o" <tytso@....edu> Cc: linux-ext4@...r.kernel.org Subject: Re: [PATCH 06/22] ext4: Calculate and verify inode checksums On Mon, Dec 05, 2011 at 11:24:06AM -0500, Ted Ts'o wrote: > On Mon, Nov 28, 2011 at 03:26:56PM -0800, Darrick J. Wong wrote: > > + /* Precompute second piece of csum */ > > + if (EXT4_HAS_RO_COMPAT_FEATURE(sb, > > + EXT4_FEATURE_RO_COMPAT_METADATA_CSUM)) { > > + __u32 csum; > > + struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb); > > + __le32 inum = cpu_to_le32(inode->i_ino); > > + __le32 gen = cpu_to_le32(inode->i_generation); > > + csum = ext4_chksum(sbi, sbi->s_uuid_csum, (__u8 *)&inum, > > + sizeof(inum)); > > + ei->i_uuid_inum_csum = ext4_chksum(sbi, csum, (__u8 *)&gen, > > + sizeof(gen)); > > + } > > Why do we include a copy of i_generation in the precomputed initial > part of the checksum? Since i_generation is in the raw, on-disk > version of the inode, what's the rationale for including it here? It > shouldn't *hurt*, but it a few extra CPU cycles, and I'm not seeing > how it helps. i_uuid_inum_csum seeds crc32c for all the metadata objects that are attached to an inode (extents, dir blocks, xattrs), not just the inode itself. That way you can ensure that a piece of metadata corresponds to a specific incarnation of an inode, not just any of the inode's incarnations since mkfs time. --D > > - Ted > -- > 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 > -- 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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