netdev - Re: [PATCH bpf 2/3] libbpf: restore umem state after socket create failure

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Message-ID: <CAJ8uoz2Om5HdaWSN6UG5Os2GMQCtJ8dRqB_QN4Lw=kbm6fEe1g@mail.gmail.com>
Date:   Fri, 26 Mar 2021 10:06:29 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Ciara Loftus <ciara.loftus@...el.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn@...nel.org>
Subject: Re: [PATCH bpf 2/3] libbpf: restore umem state after socket create failure

On Wed, Mar 24, 2021 at 3:46 PM Ciara Loftus <ciara.loftus@...el.com> wrote:
>
> If the call to socket_create fails, the user may want to retry the
> socket creation using the same umem. Ensure that the umem is in the
> same state on exit if the call failed by restoring the _save pointers
> and not unmapping the set of umem rings if those pointers are non NULL.
>
> Fixes: 2f6324a3937f ("libbpf: Support shared umems between queues and devices")
>
> Signed-off-by: Ciara Loftus <ciara.loftus@...el.com>
> ---
>  tools/lib/bpf/xsk.c | 29 ++++++++++++++++++-----------
>  1 file changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index 443b0cfb45e8..ec3c23299329 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -743,21 +743,23 @@ static struct xsk_ctx *xsk_get_ctx(struct xsk_umem *umem, int ifindex,
>         return NULL;
>  }
>
> -static void xsk_put_ctx(struct xsk_ctx *ctx)
> +static void xsk_put_ctx(struct xsk_ctx *ctx, bool unmap)
>  {
>         struct xsk_umem *umem = ctx->umem;
>         struct xdp_mmap_offsets off;
>         int err;
>
>         if (--ctx->refcount == 0) {
> -               err = xsk_get_mmap_offsets(umem->fd, &off);
> -               if (!err) {
> -                       munmap(ctx->fill->ring - off.fr.desc,
> -                              off.fr.desc + umem->config.fill_size *
> -                              sizeof(__u64));
> -                       munmap(ctx->comp->ring - off.cr.desc,
> -                              off.cr.desc + umem->config.comp_size *
> -                              sizeof(__u64));
> +               if (unmap) {
> +                       err = xsk_get_mmap_offsets(umem->fd, &off);
> +                       if (!err) {
> +                               munmap(ctx->fill->ring - off.fr.desc,
> +                                      off.fr.desc + umem->config.fill_size *
> +                               sizeof(__u64));
> +                               munmap(ctx->comp->ring - off.cr.desc,
> +                                      off.cr.desc + umem->config.comp_size *
> +                               sizeof(__u64));
> +                       }
>                 }

By not unmapping these rings we actually leave more state after a
failed socket creation. So how about skipping this logic (and
everything below) and always unmap the rings at failure as before, but
we move the fill_save = NULL and comp_save = NULL from xsk_create_ctx
to the end of xsk_socket__create_shared just before the "return 0"
where we know that the whole operation has succeeded. This way the
mappings would be redone during the next xsk_socket__create and if
someone decides not to retry (for some reason) we do not leave two
mappings behind. Would simplify things. What do you think?

>
>                 list_del(&ctx->list);
> @@ -854,6 +856,9 @@ int xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
>         struct xsk_socket *xsk;
>         struct xsk_ctx *ctx;
>         int err, ifindex;
> +       struct xsk_ring_prod *fsave = umem->fill_save;
> +       struct xsk_ring_cons *csave = umem->comp_save;
> +       bool unmap = !fsave;
>
>         if (!umem || !xsk_ptr || !(rx || tx))
>                 return -EFAULT;
> @@ -1005,7 +1010,9 @@ int xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
>                 munmap(rx_map, off.rx.desc +
>                        xsk->config.rx_size * sizeof(struct xdp_desc));
>  out_put_ctx:
> -       xsk_put_ctx(ctx);
> +       umem->fill_save = fsave;
> +       umem->comp_save = csave;
> +       xsk_put_ctx(ctx, unmap);
>  out_socket:
>         if (--umem->refcount)
>                 close(xsk->fd);
> @@ -1071,7 +1078,7 @@ void xsk_socket__delete(struct xsk_socket *xsk)
>                 }
>         }
>
> -       xsk_put_ctx(ctx);
> +       xsk_put_ctx(ctx, true);
>
>         umem->refcount--;
>         /* Do not close an fd that also has an associated umem connected
> --
> 2.17.1
>