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
