linux-kernel - Re: [PATCH bpf-next] xsk: allow remap of fill and/or completion rings

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Message-ID: <20230320134058.GM36557@unreal>
Date:   Mon, 20 Mar 2023 15:40:58 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     Nuno Gonçalves <nunog@...4.com>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] xsk: allow remap of fill and/or completion rings

On Mon, Mar 20, 2023 at 01:27:18PM +0100, Magnus Karlsson wrote:
> On Mon, 20 Mar 2023 at 12:09, Leon Romanovsky <leon@...nel.org> wrote:
> >
> > On Mon, Mar 20, 2023 at 10:53:23AM +0000, Nuno Gonçalves wrote:
> > > The remap of fill and completion rings was frowned upon as they
> > > control the usage of UMEM which does not support concurrent use.
> > > At the same time this would disallow the remap of this rings
> > > into another process.
> > >
> > > A possible use case is that the user wants to transfer the socket/
> > > UMEM ownerwhip to another process (via SYS_pidfd_getfd) and so
> 
> nit: ownership
> 
> > > would need to also remap this rings.
> > >
> > > This will have no impact on current usages and just relaxes the
> > > remap limitation.
> > >
> > > Signed-off-by: Nuno Gonçalves <nunog@...4.com>
> > > ---
> > >  net/xdp/xsk.c | 9 ++++++---
> > >  1 file changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > > index 2ac58b282b5eb..2af4ff64b22bd 100644
> > > --- a/net/xdp/xsk.c
> > > +++ b/net/xdp/xsk.c
> > > @@ -1300,10 +1300,11 @@ static int xsk_mmap(struct file *file, struct socket *sock,
> > >  {
> > >       loff_t offset = (loff_t)vma->vm_pgoff << PAGE_SHIFT;
> > >       unsigned long size = vma->vm_end - vma->vm_start;
> > > +     int state = READ_ONCE(xs->state);
> 
> Reverse Christmas Tree notation here please. Move it one line down to
> after the *xs declaration.
> 
> > >       struct xdp_sock *xs = xdp_sk(sock->sk);
> > >       struct xsk_queue *q = NULL;
> > >
> > > -     if (READ_ONCE(xs->state) != XSK_READY)
> > > +     if (!(state == XSK_READY || state == XSK_BOUND))
> >
> > This if(..) is actually:
> >  if (state != XSK_READY && state != XSK_BOUND)
> 
> Nuno had it like that to start with when he sent the patch privately
> to me, but I responded that I prefered the current one. It is easier
> to understand if read out aloud IMO. 

"Not equal" is much easier to understand than "not" of whole expression.

> Do not have any strong feelings either way since the statements are equivalent.
> 
> > Thanks