On Tue, Dec 06, 2011 at 10:19:12AM -0700, Andreas Dilger wrote: > On 2011-12-05, at 9:33, Ted Ts'o <tytso@xxxxxxx> wrote: > > Note: it's strictly speaking not necessary to mix in the group and > > s_csum_seed here. It's useful for the inode table blocks (ITB's) > > because the checksum for a particular ITB is located *in* the ITB > > itself. So if an ITB gets written to the wrong place, and in > > particular, on top of another ITB, we want to be able to know which > > cloned copy was written to the wrong place on disk. > > > > But in the case of the inode and block allocation bitmaps, the > > checksums are stored in the block group descriptors; so if the bitmap > > is written to the wrong place (and on top of another bitmap), the > > checksum will fail to verify, independent of whether we've mixed in > > the fs-specific csum seed and the group number. > > > > So I'd suggest dropping this, which will shave a few cycles off of the > > checksum calculation, and it will also simplify the code since we > > won't need this particular function. > > I wouldn't mind keeping the group just to be consistent with all of the other > checksums that are used in the filesystem, which are largely inside the > structure being checked. > > The s_uuid is definitely useful to keep as the seed because the block and > inode bitmaps are not initialized at mke2fs time with uninit_bg, and it is > possible to read a stale bitmap from disk that might belong to an earlier > instance of the filesystem in case of a failed or misplaced write of the > correct bitmap. Hmm... let's say you have bitmap B before mkfs and bitmap B' after mkfs + some file writes. B' is lost during write. It would be bad if B != B' and crc32c(B) == crc32c(B'), in which case you'd use the wrong bitmap. I suppose having the fsuuid + groupnum would probably help to make the inputs to crc32c() more distinct, which would be useful since iirc P(collision) decreases as the Hamming distance increases. I'm running a simulation to check that claim. --D > That isn't important for e2fsck, since it doesn't really use the bitmaps, but > it is important for the kernel not to use bad bitmaps and corrupt the > filesystem further. > > Cheers, Andreas-- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
