On Tue, Dec 06, 2011 at 10:19:12AM -0700, Andreas Dilger wrote:

On 2011-12-05, at 9:33, Ted Ts'o [off-list ref] wrote:

quoted

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--
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