netdev - Re: [RFC PATCH v2 04/48] net: Declare MSG_SPLICE

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Message-ID: <6425cc3c9d750_21f56920832@willemb.c.googlers.com.notmuch>
Date:   Thu, 30 Mar 2023 13:51:56 -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>,
        Chuck Lever III <chuck.lever@...cle.com>,
        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 v2 04/48] net: Declare MSG_SPLICE_PAGES internal
 sendmsg() flag

David Howells wrote:
> Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> 
> > No need to modify __sys_sendmmsg explicitly, as it ends up calling
> > __sys_sendmsg?
> > 
> > Also, sendpage does this flags masking in the internal sock_FUNC
> > helpers rather than __sys_FUNC. Might be preferable.
> 
> I was wondering whether other flags, such as MSG_BATCH should be added to the
> list.  Is it bad if userspace sets that in sendmsg()?  AF_KCM, at least, looks
> at it.

That flag was added exactly for AF_KCM. A process that explicitly
sets it might experience bad behavior (increased latency), but
there are no legacy AF_KCM applications that precede the flag.