linux-kernel - Re: [RFC PATCH 03/28] tcp: Support MSG_SPLICE

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Date:   Tue, 21 Mar 2023 00:38:46 +0000
From:   David Howells <dhowells@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        netdev@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 03/28] tcp: Support MSG_SPLICE_PAGES

Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:

> David Howells wrote:
> > Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> > 
> > > The commit message mentions MSG_SPLICE_PAGES as an internal flag.
> > > 
> > > It can be passed from userspace. The code anticipates that and checks
> > > preconditions.
> > 
> > Should I add a separate field in the in-kernel msghdr struct for such internal
> > flags?  That would also avoid putting an internal flag in the same space as
> > the uapi flags.
> 
> That would work, if no cost to common paths that don't need it.

Actually, it might be tricky.  __ip_append_data() doesn't take a msghdr struct
pointer per se.  The "void *from" argument *might* point to one - but it
depends on seeing a MSG_SPLICE_PAGES or MSG_ZEROCOPY flag, otherwise we don't
know.

Possibly this changes if sendpage goes away.

> A not very pretty alternative would be to add an an extra arg to each
> sendmsg handler that is used only when called from sendpage.
> 
> There are a few other internal MSG_.. flags, such as
> MSG_SENDPAGE_NOPOLICY. Those are all limited to sendpage, and ignored
> in sendmsg, I think. Which would explain why it was clearly safe to
> add them.

Should those be moved across to the internal flags with MSG_SPLICE_PAGES?

David