linux-kernel - Re: [PATCH net-next v5 04/16] ceph: Use sendmsg(MSG_SPLICE

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Message-ID: <3070989.1687793422@warthog.procyon.org.uk>
Date:   Mon, 26 Jun 2023 16:30:22 +0100
From:   David Howells <dhowells@...hat.com>
To:     Ilya Dryomov <idryomov@...il.com>
Cc:     dhowells@...hat.com, netdev@...r.kernel.org,
        Alexander Duyck <alexander.duyck@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Xiubo Li <xiubli@...hat.com>,
        Jeff Layton <jlayton@...nel.org>, ceph-devel@...r.kernel.org
Subject: Re: [PATCH net-next v5 04/16] ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage()

Ilya Dryomov <idryomov@...il.com> wrote:

> write_partial_message_data() is net/ceph/messenger_v1.c specific, so it
> doesn't apply here.  I would suggest squashing the two net/ceph patches
> into one since even the titles are the same.

I would, but they're now applied to net-next, so we need to patch that.

> >   * Write as much as possible.  The socket is expected to be corked,
> > - * so we don't bother with MSG_MORE/MSG_SENDPAGE_NOTLAST here.
> > + * so we don't bother with MSG_MORE here.
> >   *
> >   * Return:
> > - *   1 - done, nothing (else) to write
> > + *  >0 - done, nothing (else) to write
> 
> It would be nice to avoid making tweaks like this to the outer
> interface as part of switching to a new internal API.

Ok.  I'll change that and wrap the sendmsg in a loop.  Though, as I asked in
an earlier reply, why is MSG_DONTWAIT used here?

> > +       if (WARN_ON(!iov_iter_is_bvec(&con->v2.out_iter)))
> > +               return -EINVAL;
> 
> Previously, this WARN_ON + error applied only to the "try sendpage"
> path.  There is a ton of kvec usage in net/ceph/messenger_v2.c, so I'm
> pretty sure that placing it here breaks everything.

This should have been removed as MSG_SPLICE_PAGES now accepts KVEC and XARRAY
iterators also.

Btw, is it feasible to use con->v2.out_iter_sendpage to apply MSG_SPLICE_PAGES
to the iterator to be transmitted as a whole?  It seems to be set depending on
iterator type.

David