[PATCH v2 21/23] xfs: handle merkle tree block size != fs blocksize != PAGE_SIZE

[Eric Biggers]

On Wed, Apr 05, 2023 at 09:38:47AM -0700, Darrick J. Wong wrote:
> > The merkle tree pages are dropped after verification. When page is
> > dropped xfs_buf is marked as verified. If fs-verity wants to
> > verify again it will get the same verified buffer. If buffer is
> > evicted it won't have verified state.
> > 
> > So, with enough memory pressure buffers will be dropped and need to
> > be reverified.
> 
> Please excuse me if this was discussed and rejected long ago, but
> perhaps fsverity should try to hang on to the merkle tree pages that
> this function returns for as long as possible until reclaim comes for
> them?
> 
> With the merkle tree page lifetimes extended, you then don't need to
> attach the xfs_buf to page->private, nor does xfs have to extend the
> buffer cache to stash XBF_VERITY_CHECKED.

Well, all the other filesystems that support fsverity (ext4, f2fs, and btrfs)
just cache the Merkle tree pages in the inode's page cache.  It's an approach
that I know some people aren't a fan of, but it's efficient and it works.

We could certainly think about moving to a design where fs/verity/ asks the
filesystem to just *read* a Merkle tree block, without adding it to a cache, and
then fs/verity/ implements the caching itself.  That would require some large
changes to each filesystem, though, unless we were to double-cache the Merkle
tree blocks which would be inefficient.

So it feels like continuing to have the filesystem (not fs/verity/) be
responsible for the cache is the best way to allow XFS to do things a bit
differently, without regressing the other filesystems.

I'm interested in hearing any other proposals, though.

- Eric