netdev - Re: [RFC PATCH 27/28] sunrpc: Use sendmsg(MSG_SPLICE

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Message-ID: <809995.1678990010@warthog.procyon.org.uk>
Date:   Thu, 16 Mar 2023 18:06:50 +0000
From:   David Howells <dhowells@...hat.com>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Jeffrey Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Anna Schumaker <anna@...nel.org>,
        Charles Edward Lever <chuck.lever@...cle.com>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: [RFC PATCH 27/28] sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage

Trond Myklebust <trondmy@...merspace.com> wrote:

> 1) This is code that is common to the client and the server. Why are we
> adding unused 3 bvec slots to every client RPC call?

Fair point, but I'm trying to avoid making four+ sendmsg calls in nfsd rather
than one.

> 2) It obfuscates the existence of these bvec slots.

True, it'd be nice to find a better way to do it.  Question is, can the client
make use of MSG_SPLICE_PAGES also?

> 3) knfsd may use splice_direct_to_actor() in order to avoid copying the page
> cache data into private buffers (it just takes a reference to the
> pages). Using MSG_SPLICE_PAGES will presumably require it to protect those
> pages against further writes while the socket is referencing them.

Upstream sunrpc is using sendpage with TCP.  It already has that issue.
MSG_SPLICE_PAGES is a way of doing sendpage through sendmsg.

David