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From:   David Howells <dhowells@redhat.com>
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References: <20230622111234.23aadd87@kernel.org> <20230620145338.1300897-1-dhowells@redhat.com> <20230620145338.1300897-2-dhowells@redhat.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     dhowells@redhat.com, netdev@vger.kernel.org,
        Alexander Duyck <alexander.duyck@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        David Ahern <dsahern@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Menglong Dong <imagedong@tencent.com>
Subject: Re: [PATCH net-next v3 01/18] net: Copy slab data for sendmsg(MSG_SPLICE_PAGES)
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Date:   Thu, 22 Jun 2023 20:40:43 +0100
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Jakub Kicinski <kuba@kernel.org> wrote:

> > If sendmsg() is passed MSG_SPLICE_PAGES and is given a buffer that con=
tains
> > some data that's resident in the slab, copy it rather than returning E=
IO.
> =

> How did that happen? I thought MSG_SPLICE_PAGES comes from former
> sendpage users and sendpage can't operate on slab pages.

Some of my patches, take the siw one for example, now aggregate all the bi=
ts
that make up a message into a single sendmsg() call, including any protoco=
l
header and trailer in the same bio_vec[] as the payload where before it wo=
uld
have to do, say, sendmsg+sendpage+sendpage+...+sendpage+sendmsg.

I'm trying to make it so that I make the minimum number of sendmsg calls
(ie. 1 where possible) and the loop that processes the data is inside of t=
hat.
This offers the opportunity, at least in the future, to append slab data t=
o an
already-existing private fragment in the skbuff.

> The locking is to local_bh_disable(). Does the milliont^w new frag
> allocator have any additional benefits?

It is shareable because it does locking.  Multiple sockets of multiple
protocols can share the pages it has reserved.  It drops the lock around c=
alls
to the page allocator so that GFP_KERNEL/GFP_NOFS can be used with it.

Without this, the page fragment allocator would need to be per-socket, I
think, or be done further up the stack where the higher level drivers woul=
d
have to have a fragment bucket per whatever unit they use to deal with the
lack of locking.

Doing it here makes cleanup simpler since I just transfer my ref on the
fragment to the skbuff frag list and it will automatically be cleaned up w=
ith
the skbuff.

Willy suggested that I just allocate a page for each thing I want to copy,=
 but
I would rather not do that for, say, an 8-byte bit of protocol data.

David