linux-kernel - Re: [RFC PATCH 00/31] netfs: [WIP] Allow the use of MSG_SPLICE

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Message-ID: <2572846.1754547848@warthog.procyon.org.uk>
Date: Thu, 07 Aug 2025 07:24:08 +0100
From: David Howells <dhowells@...hat.com>
To: Stefan Metzmacher <metze@...ba.org>
Cc: dhowells@...hat.com, Steve French <sfrench@...ba.org>,
    Paulo Alcantara <pc@...guebit.org>,
    Shyam Prasad N <sprasad@...rosoft.com>, Tom Talpey <tom@...pey.com>,
    Wang Zhaolong <wangzhaolong@...weicloud.com>,
    Mina Almasry <almasrymina@...gle.com>, linux-cifs@...r.kernel.org,
    linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/31] netfs: [WIP] Allow the use of MSG_SPLICE_PAGES and use netmem allocator

Stefan Metzmacher <metze@...ba.org> 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.

However, this is meant to be an intermediate step.  I actually want to
assemble the fragment list in a bvecq in the smb_create_request() as called by
the PDU encoders, with everything aligned for crypto so that the crypto layer
doesn't copy it also.  But cleaning up the transport first should hopefully
reduce the size of the later patches.

David