On Thu, Apr 06, 2023 at 09:37:53AM +1000, Dave Chinner wrote: > On Wed, Apr 05, 2023 at 10:54:06PM +0000, Eric Biggers wrote: > > 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. > > Boolean flag from the caller. > > > 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? > > Not vmalloc'ed, but vmapped. we allocate the pages individually, but > then call vm_map_page() to present the higher level code with a > single contiguous memory range if it is a multi-page buffer. > > We do have the backing info held in the buffer, and that's what we > use for IO. If fsverity needs a page based scatter/gather list > for hardware offload, it could ask the filesystem to provide it > for that given buffer... > > > 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. > > True, but we are very unlikely to be using XFS on such small > systems and I don't think we really care about XFS performance on > android sized systems, either. > FYI, I've sent an RFC patch that converts fs/verity/ from ahash to shash: https://lore.kernel.org/r/20230406003714.94580-1-ebiggers@xxxxxxxxxx It would be great if we could do that. But I need to get a better sense for whether anyone will complain... - Eric
