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b=L2Up9Zp/7tLPcWy426HJxpyxK5jrLwIKsJh/I7EJrRqcMVmPW2D3+n64+taNy6XZD ZwdSTJ3RON53sNMh72dYcdBngzi+//cBXjPCP2TR0P2pLDzeSCUSNIp2+t/UrFFG+f l5kJXpa9fMPly8p1SjmScFTJJ15uQW2Zu81INbFDgDLPz5mtcYEx1uNknRioWz8unX U0un4cVpgXwBSRBNNymxkAycgiZmzK55Txhoa8kYnHkznYtABdxBsIRTGYEIQQqIYB F0JhkZF7PR8dLeyUevRBWaTjlMrnD/sVqHpvTV1ougk40oJuqlpF6GsQKvj5/BOxGQ cyjSy6gZLovcA== Message-ID: Date: Wed, 3 Jun 2026 09:50:15 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2 To: Eric Biggers Cc: Andrew Morton , Steven Rostedt , 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> <821ed41e-5b2f-4d17-aeb2-71b0361f8e7f@kernel.org> <20260602184440.GB2503276@google.com> 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-Rspamd-Queue-Id: 08420A0013 X-Rspam-User: X-Stat-Signature: bgfz4pn181xntz9haba766tuj7eopowu X-Rspamd-Server: rspam09 X-HE-Tag: 1780473022-757753 X-HE-Meta: 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 26ZbkGEG R5qFlPUT/hgn2e3GiGFrkI/r5C/xgf8sGg/iSAqPDVHKCPoS+ECpiQfZpi8IDq31LY24OdrcaWYZmA0zj3Lr2Jx5BvM6v7HSeqNEQdK1RCg0AHSSJiM8HY7KcBZkRQB2hnxFYGDFN8ePemCJbHY2NcZB1pA2Ahdw3V1C65eY1MqSK+3/1fqxZZ3K6bJPP81I/wkVe0Ke3yd7MExANKrzmNxLg1sSRVxM+QfoLtr1XW2tPh+Krsfuz7EdYnLnra5bn9eqPvBnUCnDD7bLnReGStix7Ig684i9HAN83DeeYD19WCPj/x+CC9FbUwQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/2/26 20:44, Eric Biggers wrote: > On Tue, Jun 02, 2026 at 10:25:06AM +0200, David Hildenbrand (Arm) wrote: >> On 6/2/26 02:28, Andrew Morton wrote: >>> >>> >>> 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 ... :) > > AF_ALG is a mistake and isn't commonly used. Using a userspace crypto > library is faster and is what almost everyone does anyway, as it avoids > the syscall overhead. There are many other issues with AF_ALG as well. > > 7.2 will mark AF_ALG as deprecated, mostly remove AF_ALG's zero-copy > support, and remove AF_ALG's async I/O support: > > https://lore.kernel.org/linux-crypto/20260430011544.31823-1-ebiggers@kernel.org/ > https://lore.kernel.org/linux-crypto/20260504225328.25356-1-ebiggers@kernel.org/ > https://lore.kernel.org/linux-crypto/20260523-af-alg-harden-v1-0-c76755c3a5c5@gmail.com/ > > In practice, the programs that are keeping Linux distros from disabling > AF_ALG in their kconfig outright are just iwd, cryptsetup, and bluez. > They use AF_ALG just because it was mistakenly thought to be easier than > using a userspace crypto library. They don't need maximum performance, > nor do they use vmsplice, splice, or sendfile. > > There is other highly niche code out there that does implement the > AF_ALG + vmsplice + splice thing, e.g. libkcapi. But it's just not > enough of a reason to keep zero-copy support, especially considering > that AF_ALG has always been the wrong solution in the first place. The > fallback to copying the data is fine for this deprecated API. Cool, thanks for sharing that Eric! -- Cheers, David