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

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Message-ID: <c213ace1-7845-4454-a746-8a5926ab41d0@samba.org>
Date: Thu, 7 Aug 2025 07:23:00 +0200
From: Stefan Metzmacher <metze@...ba.org>
To: David Howells <dhowells@...hat.com>, Steve French <sfrench@...ba.org>
Cc: 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

Hi David,

> The aim is to build up a list of fragments for each request using a bvecq.
> These form a segmented list and can be spliced together when assembling a
> compound request.  The segmented list can then be passed to sendmsg() with
> MSG_SPLICE_PAGES in a single call, thereby only having a single loop (in
> the TCP stack) to shovel data, not loops-over-loops.  Possibly we can
> dispense with corking also, provided we can tell TCP to flush the record
> boundaries.  (Note that this also simplifies smbd_send() for RDMA).

I didn't look at the patches in detail, but I like the simplifications
for the transport layer and that hopefully allows me to
move smbd_send() behind sendmsg() with MSG_SPLICE_PAGES in the end.

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?

Thanks!
metze