Subject: RE: [PATCH bpf 2/3] libbpf: restore umem state after socket create failure

From: "Loftus, Ciara" <[email-masked]>
Date: Fri, 26 Mar 2021 14:56:30 +0000

> 
> On Wed, Mar 24, 2021 at 3:46 PM Ciara Loftus <[email-masked]>
> 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 <[email-masked]>
> > ---
> >  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

In the case of the _save rings, the maps existed before the call to
xsk_socket__create. They were created during xsk_umem__create.
So we should preserve these maps in event of failure.
I was using the wrong condition to trigger the unmap in v1 however.
We should unmap 'fill' only if
        umem->fill_save != fill
I will update this in a v2.

> 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

I think moving these still makes sense and will add this in the next rev.

Thanks for the feedback and suggestions!

Ciara

> 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
> >