linux-kernel - Re: [PATCH bpf-next V2] xsk: allow remap of fill and/or completion rings

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Message-ID: <CAJ8uoz2N4M+FB-ijzTrVm+91yhtqfKKwmPkxjefJrmSeJOocbg@mail.gmail.com>
Date:   Tue, 21 Mar 2023 12:13:24 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Nuno Gonçalves <nunog@...4.com>
Cc:     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 V2] xsk: allow remap of fill and/or completion rings

On Mon, 20 Mar 2023 at 21:54, Nuno Gonçalves <nunog@...4.com> 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 these rings
> into another process.
>
> A possible use case is that the user wants to transfer the socket/
> UMEM ownership to another process (via SYS_pidfd_getfd) and so
> would need to also remap these rings.
>
> This will have no impact on current usages and just relaxes the
> remap limitation.

Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>

> 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..e2571ec067526 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -1301,9 +1301,10 @@ 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;
>         struct xdp_sock *xs = xdp_sk(sock->sk);
> +       int state = READ_ONCE(xs->state);
>         struct xsk_queue *q = NULL;
>
> -       if (READ_ONCE(xs->state) != XSK_READY)
> +       if (state != XSK_READY && state != XSK_BOUND)
>                 return -EBUSY;
>
>         if (offset == XDP_PGOFF_RX_RING) {
> @@ -1314,9 +1315,11 @@ static int xsk_mmap(struct file *file, struct socket *sock,
>                 /* Matches the smp_wmb() in XDP_UMEM_REG */
>                 smp_rmb();
>                 if (offset == XDP_UMEM_PGOFF_FILL_RING)
> -                       q = READ_ONCE(xs->fq_tmp);
> +                       q = READ_ONCE(state == XSK_READY ? xs->fq_tmp :
> +                                                          xs->pool->fq);
>                 else if (offset == XDP_UMEM_PGOFF_COMPLETION_RING)
> -                       q = READ_ONCE(xs->cq_tmp);
> +                       q = READ_ONCE(state == XSK_READY ? xs->cq_tmp :
> +                                                          xs->pool->cq);
>         }
>
>         if (!q)
> --
> 2.40.0
>