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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <6413675eedcfa_33bc9d208e9@willemb.c.googlers.com.notmuch>
References: <6413675eedcfa_33bc9d208e9@willemb.c.googlers.com.notmuch> <641361cd8d704_33b0cc20823@willemb.c.googlers.com.notmuch> <20230316152618.711970-1-dhowells@redhat.com> <20230316152618.711970-4-dhowells@redhat.com> <811534.1678992280@warthog.procyon.org.uk>
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     dhowells@redhat.com, Matthew Wilcox <willy@infradead.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jeff Layton <jlayton@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH 03/28] tcp: Support MSG_SPLICE_PAGES
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Date:   Tue, 21 Mar 2023 00:38:46 +0000
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Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:

> David Howells wrote:
> > Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:
> > =

> > > The commit message mentions MSG_SPLICE_PAGES as an internal flag.
> > > =

> > > It can be passed from userspace. The code anticipates that and check=
s
> > > 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 spa=
ce 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 st=
ruct
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 do=
n'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