Re: [PATCH v2 01/12] fs-verity: add a documentation file

[Christoph Hellwig]

On Mon, Dec 17, 2018 at 12:00:39PM -0800, Darrick J. Wong wrote:
> FWIW, if I were (hypothetically) working on an xfs implementation, I
> likely would have settled on passing a reference to a merkle tree
> through a (fd, length) pair, because that allows us plenty of options
> on the back end:
> 
> b) we could remap the tree into a new inode fork for merkle trees, or
> a) remap it as posteof blocks like ext4/f2fs does, or
> c) remap the blocks into the attribute fork as an (unusually large)
> extended attribute value.
>
> If the merkle_fd isn't on the same filesystem as the fd we could at
> least use generic_copy_file_range (i.e. page cache copying) to land the
> merkle tree wherever we want.

I think the fd would have to be on the same fs for this interface to
make sense. But it could be an O_TMPFILE one.  And given that ext4
already supports a variant of swapext this interface should also work
with the existing ext4 on disk format.