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Date:   Thu, 30 Mar 2023 13:51:56 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To:     David Howells <dhowells@redhat.com>,
        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>,
        Chuck Lever III <chuck.lever@oracle.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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 <20230329141354.516864-1-dhowells@redhat.com>
 <20230329141354.516864-5-dhowells@redhat.com>
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Subject: Re: [RFC PATCH v2 04/48] net: Declare MSG_SPLICE_PAGES internal
 sendmsg() flag
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David Howells wrote:
> Willem de Bruijn <willemdebruijn.kernel@gmail.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.