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b=QlNXRHxSiVLWRkqR3Q83ejTs3EHdyrR1prTvQZYTL8YtFs0PR52piVz8fyJEJM2uC hJ0TWK8wgrljJbxRonpFInx6SFOlMhVMZDhiW7rHu/6VGdbp/Oy15a+GKR94qHGpTS VCQM4Fhfkz0Sa0rxZoUqWUjjPxcva4ax7udZsjeOqX0J3reXgreOaDMiaJvCPgC0j2 meOw6D3lKrik3jhF+vZJn3oeQsp7CD5E41F2hw99M6ee25Iou8YF8cZwu/HsyhxEqf FWnXd70Qv5lqK5b7AmTLci9xmpmuJwDz8nTPaMds7BQqU2ZMFJhKvEBeT4RFna+atK 6NwEOEfbTStiw== Message-ID: <821ed41e-5b2f-4d17-aeb2-71b0361f8e7f@kernel.org> Date: Tue, 2 Jun 2026 10:25:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2 To: Andrew Morton , Steven Rostedt Cc: Al Viro , Linus Torvalds , Christian Brauner , Askar Safin , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, netdev@vger.kernel.org, Matthew Wilcox , Jens Axboe , Christoph Hellwig , David Howells , Pedro Falcato , Miklos Szeredi , patches@lists.linux.dev, linux-fsdevel@vger.kernel.org, Jan Kara References: <20260531010107.1953702-1-safinaskar@gmail.com> <20260601-enthusiasmus-canceln-anlehnen-0e62317a9784@brauner> <20260601173325.GH2636677@ZenIV> <20260601160455.2c187574@gandalf.local.home> <20260601172825.a51a588ec1c32617a0e12d78@linux-foundation.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 6DE2E20004 X-Stat-Signature: jhbehme1pm19erg3qmmzxe4k5cydz8tn X-HE-Tag: 1780388712-191981 X-HE-Meta: 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 PLlxFkJf 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/2/26 02:28, Andrew Morton wrote: > On Mon, 1 Jun 2026 16:04:55 -0400 Steven Rostedt wrote: > >> On Mon, 1 Jun 2026 18:33:25 +0100 >> Al Viro wrote: >> >>> >>> >>> FUSE might be interesting - fuse_dev_splice_read() and its ilk. >>> Communications between the kernel and fuse server at least used to >>> seriously want that, so that would be one place to look for unhappy >>> userland... >>> >>> splice-related logics in fs/fuse/dev.c is interesting; another place >>> like this is kernel/trace/, but I'm less familiar with that one. >>> >>> rostedt Cc'd (miklos already had been) >> >> Thanks for the Cc. The tracing ring buffer was specifically made to be used >> by splice and the libtracefs has a lot of code to use it as well. As >> reading the ring buffer literally swaps out the write portion with a blank >> read portion, that portion (sub-buffer) is used to be directly fed into >> splice, providing a zero-copy of the trace data from the write of the event >> to going into a file. >> >> trace-cmd defaults to using splice to copy the tracing ring buffer directly >> into files to avoid as much copying during live recordings as possible. >> >> Whatever changes we make, I would like to make sure there's no regressions >> in performance of trace-cmd record. > > Well yes, The patchset seems sensible from a quality POV. But to make > a decision we should first have a decent understanding of its downside > impact. I guess most (all?) of us ... dislike ... vmsplice(), so trying to remove it entirely is certainly very appealing ... > > I haven't seen a description of that impact in the discussion thus far. > And that description is owed, please. > > I assume a small number of specialized applications are using > vmsplice() to great effect? What are those applications? What is the > impact of this change? I did some digging, and the kernel crypto API documents using splice/vmsplice for zero-copy[1] and libkcapi [2]. I did not find performance numbers, how much vmsplice/splice actually gives us. Playing with the kcapi-speed tool [3] (specifying --vmsplice vs. --sendmsg) doesn't really reveal a big difference at least on my notebook. Not sure if the parameters I specify are reasonable. I don't know whether downgrading vmsplice to preadv2/pwritev2 would perform significantly worse than sendmsg ... and I don't know what the default would usually be (default to vmsplice or sendmsg). I might try finding some time to play with it more, but I doubt it, so if anybody else has time ... :) I'll note that we have a bunch of selftests (mostly around COW handling) that rely on vmsplice to test R/O pinning behavior. For R/W pinning, we can use iouring fixed buffers easily. The only alternative for R/O pinning is using the gup_test infrastructure that needs to be compiled into the kernel, unfortunately ... So we'll have to adjust some tests there to use a different interface. I'm sure I can find someone to work on that once this change here landed and doesn't have to be yanked immediately again. [1] https://www.kernel.org/doc/html/latest/crypto/userspace-if.html [2] https://github.com/smuellerDD/libkcapi/blob/master/lib/kcapi-kernel-if.c [3] https://github.com/smuellerDD/libkcapi/tree/master/speed-test -- Cheers, David