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

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Message-ID: <6413675eedcfa_33bc9d208e9@willemb.c.googlers.com.notmuch>
Date:   Thu, 16 Mar 2023 15:00:46 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     David Howells <dhowells@...hat.com>,
        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

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.

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.