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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <40d1f78c-d437-4ab4-8e5e-8708af6486ab@samba.org> References: <40d1f78c-d437-4ab4-8e5e-8708af6486ab@samba.org> <20250806203705.2560493-1-dhowells@redhat.com> <2572846.1754547848@warthog.procyon.org.uk> To: Stefan Metzmacher Cc: dhowells@redhat.com, Steve French , Paulo Alcantara , Shyam Prasad N , Tom Talpey , Wang Zhaolong , Mina Almasry , linux-cifs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 00/31] netfs: [WIP] Allow the use of MSG_SPLICE_PAGES and use netmem allocator Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <2573494.1754550778.1@warthog.procyon.org.uk> Date: Thu, 07 Aug 2025 08:12:58 +0100 Message-ID: <2573495.1754550778@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Stefan Metzmacher wrote: > >> So the current situation is that we memcpy (at least) in sendmsg() > >> and with your patches we do a memcpy higher in the stack, but then > >> use MSG_SPLICE_PAGES in order to do it twice. Is that correct? > > Not twice, no. MSG_SPLICE_PAGES allows sendmsg() to splice the supplied > > pages > > into the sk_buffs directly, thereby avoiding a copy in the TCP layer and > > cutting out the feeder loop in cifs. > > Yes, and we must be careful to not touch the pages after > calling sendmsg(MSG_SPLICE_PAGES). Until we get a response from the server, yes, but for the protocol info that shouldn't be an issue. And if we're going to encrypt, we'll have to do a copy anyway for something like Write, but we can get the encryption algo to do that for us by giving it a separate destination buffer. > And unlike MSG_ZEROCOPY tcp_sendmsg_locked() has no > no struct ubuf_info *uarg when MSG_SPLICE_PAGES is used > and there's no way to know when the pages are no longer > used by the tcp stack. Correct (and this is something we'll need to address), but for the moment we can rely on page refcounts. MSG_SPLICE_PAGES takes a ref on each page - which is why you can't use it with slab memory. However, if we pass in netmem-allocated memory, that works by refcounting, so that should work. > Can you explain how/where we allocate the memory and where > we unreference it in the caller of sendmsg(MSG_SPLICE_PAGES). Currently, we allocate the buffer in fs/netfs/buffer.c in netfs_alloc_bvecq_buffer(). That just bulk allocates a bunch of pages and adds them into a bvecq. As they're untyped pages, we can use the refcount. I want to allocate netmem instead, but I haven't done that yet. We then call sendmsg(MSG_SPLICE_PAGES) and then drop our ref on the pages. TCP will have taken its own ref which it will drop in due course when the skbuffs are cleaned up. David