Re: [RFC] ext4 metadata checksumming design

Andreas Dilger
2011-08-22T18:11:27Z

On 2011-08-16, at 9:25 PM, Darrick J. Wong wrote:
> I've created a page on the ext4 wiki outlining the patchset that I'm working on
> to add metadata checksumming to ext4. The page can be found at this address:
> https://ext4.wiki.kernel.org/index.php/Ext4_Metadata_Checksums

Darrick,
I just had a look though this document, and it looks pretty good. It does
need to be updated to reflect that the inode checksum now covers the full
inode size, which is already mentioned in the "Extended Attributes" section.

> For the most part, the metadata objects in ext4 actually have enough space to
> squeeze in a 32-bit checksum; it was trivially easy to find a spot in the
> superblock, the extent tree, extended attribute blocks, and the inode. Those
> pieces are already done and in my tree, but the patchset as a whole is being
> held up by the second class of metadata objects.

For the group descriptor checksum and inode/block bitmap checksums with
32-byte group descriptors it makes sense to truncate the CRC32c checksum
and store the low bits of the checksum in the existing 16-bit fields, and
the high bits in extended 16-bit fields.

As a follow on, it probably also makes sense to test with a < 2^32 block
filesystem with a 64-byte group descriptor. That would give enough room
for 32-bit checksums even on smaller filesystems, and would also help
facilitate resizing filesystems from < 2^32 blocks to > 2^32 blocks in
the future. That _may_ just be as easy as formatting with "-O 64bit"
on a < 2^32 block filesystem, but I don't know how much that has been
tested.

> That second class of objects are the ones that required a bit of work:
>
> - Directory blocks have an "unused" 12-byte directory entry at the very end of
> the block; 8 bytes of header are followed by a 32-bit checksum. This can be
> taken care of as part of directory rebuilding in e2fsck/rehash.c.
>
> - HTree blocks had to have the dx_entry limit reduced by 1 to accomodate a
> checksum. This is also taken care of during e2fsck directory rebuild.
>
> - Extended attribute blocks that are stored in the inode table -- the h_magic
> field is written by the kernel, but neither the kernel nor e2fsprogs ever
> actually read this field. The field could be reused to checksum the extra
> space since (as far as I can tell) EAs are the only user of that empty space.
>
> Other miscellany:
>
> - e2fsprogs had to be converted to always work with ext2_inode_large.
>
> - Various bugs in the htree code....
>
> I hope to have a first draft of the kernel/e2fsprogs patches out on the mailing
> list in a week or two, or at least before LPC next month. Still on my todo
> list is superblocks, EAs, changing the jbd2 checksum, and rigorous testing on
> powerpc.
>
> Please have a look at the design document and please feel free to suggest any
> changes.
>
> --D