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

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Date:   Mon, 26 Jun 2023 18:07:18 +0200
From:   Ilya Dryomov <idryomov@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     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()

On Mon, Jun 26, 2023 at 5:30 PM David Howells <dhowells@...hat.com> wrote:
>
> 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.

I don't see a problem with that given that the patches themselves have
major issues (i.e. it's not just a commit message/title nit).

>
> > >   * 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?

See my reply there.

>
> > > +       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.

I'm not sure I understand what you mean by "transmitted as a whole".
con->v2.out_iter_sendpage is set only when zerocopy is desired.  If the
underlying data is not guaranteed to remain stable, zerocopy behavior
is not safe.

Thanks,

                Ilya