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

[Eric Biggers]

On Thu, Apr 06, 2023 at 08:26:46AM +1000, Dave Chinner wrote:
> > 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.
> 
> No, that's unnecessary.
> 
> All we need if for fsverity to require filesystems to pass it byte
> addressable data buffers that are externally reference counted. The
> filesystem can take a page reference before mapping the page and
> passing the kaddr to fsverity, then unmap and drop the reference
> when the merkle tree walk is done as per Andrey's new drop callout.
> 
> fsverity doesn't need to care what the buffer is made from, how it
> is cached, what it's life cycle is, etc. The caching mechanism and
> reference counting is entirely controlled by the filesystem callout
> implementations, and fsverity only needs to deal with memory buffers
> that are guaranteed to live for the entire walk of the merkle
> tree....

Sure.  Just a couple notes:

First, fs/verity/ does still need to be able to tell whether the buffer is newly
instantiated or not.

Second, fs/verity/ uses the ahash API to do the hashing.  ahash is a
scatterlist-based API.  Virtual addresses can still be used (see sg_set_buf()),
but the memory cannot be vmalloc'ed memory, since virt_to_page() needs to work.
Does XFS use vmalloc'ed memory for these buffers?

BTW, converting fs/verity/ from ahash to shash is an option; I've really never
been a fan of the scatterlist-based crypto APIs!  The disadvantage of doing
this, though, would be that it would remove support for all the hardware crypto
drivers.  That *might* actually be okay, as that approach to crypto acceleration
has mostly fallen out of favor, in favor of CPU-based acceleration.  But I do
worry about e.g. someone coming out of the woodwork and saying they need to use
fsverity on a low-powered ARM board that has a crypto accelerator like CAAM, and
they MUST use their crypto accelerator to get acceptable performance.

- Eric