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[49.181.91.157]) by smtp.gmail.com with ESMTPSA id m37-20020a635825000000b00502dc899394sm9641716pgb.66.2023.04.05.16.37.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 05 Apr 2023 16:37:57 -0700 (PDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1pkChN-00HVv9-Mh; Thu, 06 Apr 2023 09:37:53 +1000 Date: Thu, 6 Apr 2023 09:37:53 +1000 To: Eric Biggers Subject: Re: [PATCH v2 21/23] xfs: handle merkle tree block size != fs blocksize != PAGE_SIZE Message-ID: <20230405233753.GU3223426@dread.disaster.area> References: <20230404145319.2057051-1-aalbersh@redhat.com> <20230404145319.2057051-22-aalbersh@redhat.com> <20230404163602.GC109974@frogsfrogsfrogs> <20230405160221.he76fb5b45dud6du@aalbersh.remote.csb> <20230405163847.GG303486@frogsfrogsfrogs> <20230405222646.GR3223426@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-BeenThere: linux-erofs@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Development of Linux EROFS file system List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Dave Chinner via Linux-erofs Reply-To: Dave Chinner Cc: fsverity@lists.linux.dev, linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, agruenba@redhat.com, "Darrick J. Wong" , Andrey Albershteyn , linux-f2fs-devel@lists.sourceforge.net, hch@infradead.org, cluster-devel@redhat.com, dchinner@redhat.com, rpeterso@redhat.com, jth@kernel.org, linux-erofs@lists.ozlabs.org, damien.lemoal@opensource.wdc.com, linux-btrfs@vger.kernel.org Errors-To: linux-erofs-bounces+linux-erofs=archiver.kernel.org@lists.ozlabs.org Sender: "Linux-erofs" 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. Cheers, Dave. -- Dave Chinner david@fromorbit.com