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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <8F8B62FD-0F16-4383-BB34-97E850DAA7AF@hammerspace.com>
References: <8F8B62FD-0F16-4383-BB34-97E850DAA7AF@hammerspace.com> <3DFBF27C-A62B-4AFE-87FD-3DF53FC39E8E@hammerspace.com> <20230316152618.711970-1-dhowells@redhat.com> <20230316152618.711970-28-dhowells@redhat.com> <754534.1678983891@warthog.procyon.org.uk>
To:     Trond Myklebust <trondmy@hammerspace.com>
Cc:     dhowells@redhat.com, Matthew Wilcox <willy@infradead.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>,
        Jeffrey Layton <jlayton@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Anna Schumaker <anna@kernel.org>,
        Charles Edward Lever <chuck.lever@oracle.com>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: [RFC PATCH 27/28] sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage
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Date:   Thu, 16 Mar 2023 18:06:50 +0000
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Trond Myklebust <trondmy@hammerspace.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