netdev - Re: [PATCH bpf-next v2 08/10] xsk: Support UMEM chunk_size > PAGE

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Date:   Tue, 4 Apr 2023 11:29:29 +0200
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Kal Cutter Conley <kal.conley@...tris.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>,
        Jonathan Corbet <corbet@....net>,
        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-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 08/10] xsk: Support UMEM chunk_size > PAGE_SIZE

On Tue, 4 Apr 2023 at 10:15, Kal Cutter Conley <kal.conley@...tris.com> wrote:
>
> > Is not the max 64K as you test against XDP_UMEM_MAX_CHUNK_SIZE in
> > xdp_umem_reg()?
>
> The absolute max is 64K. In the case of HPAGE_SIZE < 64K, then it
> would be HPAGE_SIZE.

Is there such a case when HPAGE_SIZE would be less than 64K? If not,
then just write 64K.

> > > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> > > index e96a1151ec75..ed88880d4b68 100644
> > > --- a/include/net/xdp_sock.h
> > > +++ b/include/net/xdp_sock.h
> > > @@ -28,6 +28,9 @@ struct xdp_umem {
> > >         struct user_struct *user;
> > >         refcount_t users;
> > >         u8 flags;
> > > +#ifdef CONFIG_HUGETLB_PAGE
> >
> > Sanity check: have you tried compiling your code without this config set?
>
> Yes. The CI does this also on one of the platforms (hence some of the
> bot errors in v1).

Perfect!

> > >  static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
> > >  {
> > > +#ifdef CONFIG_HUGETLB_PAGE
> >
> > Let us try to get rid of most of these #ifdefs sprinkled around the
> > code. How about hiding this inside xdp_umem_is_hugetlb() and get rid
> > of these #ifdefs below? Since I believe it is quite uncommon not to
> > have this config enabled, we could simplify things by always using the
> > page_size in the pool, for example. And dito for the one in struct
> > xdp_umem. What do you think?
>
> I used #ifdef for `page_size` in the pool for maximum performance when
> huge pages are disabled. We could also not worry about optimizing this
> uncommon case though since the performance impact is very small.
> However, I don't find the #ifdefs excessive either.

Keep them to a minimum please since there are few of them in the
current code outside of some header files. And let us assume that
CONFIG_HUGETLB_PAGE is the common case.

> > > +static void xp_check_dma_contiguity(struct xsk_dma_map *dma_map, u32 page_size)
> > >  {
> > > -       u32 i;
> > > +       u32 stride = page_size >> PAGE_SHIFT; /* in order-0 pages */
> > > +       u32 i, j;
> > >
> > > -       for (i = 0; i < dma_map->dma_pages_cnt - 1; i++) {
> > > -               if (dma_map->dma_pages[i] + PAGE_SIZE == dma_map->dma_pages[i + 1])
> > > -                       dma_map->dma_pages[i] |= XSK_NEXT_PG_CONTIG_MASK;
> > > -               else
> > > -                       dma_map->dma_pages[i] &= ~XSK_NEXT_PG_CONTIG_MASK;
> > > +       for (i = 0; i + stride < dma_map->dma_pages_cnt;) {
> > > +               if (dma_map->dma_pages[i] + page_size == dma_map->dma_pages[i + stride]) {
> > > +                       for (j = 0; j < stride; i++, j++)
> > > +                               dma_map->dma_pages[i] |= XSK_NEXT_PG_CONTIG_MASK;
> > > +               } else {
> > > +                       for (j = 0; j < stride; i++, j++)
> > > +                               dma_map->dma_pages[i] &= ~XSK_NEXT_PG_CONTIG_MASK;
> > > +               }
> >
> > Still somewhat too conservative :-). If your page size is large you
> > will waste a lot of the umem.  For the last page mark all the 4K
> > "pages" that cannot cross the end of the umem due to the max size of a
> > packet with the XSK_NEXT_PG_CONTIG_MASK bit. So you only need to add
> > one more for-loop here to mark this, and then adjust the last for-loop
> > below so it only marks the last bunch of 4K pages at the end of the
> > umem as not contiguous.
>
> I don't understand the issue. The XSK_NEXT_PG_CONTIG_MASK bit is only
> looked at if the descriptor actually crosses a page boundary. I don't
> think the current implementation wastes any UMEM.

I stand corrected. You do not waste any space, so please ignore.