linux-kernel - Re: [PATCH bpf-next v5 2/4] xsk: Support UMEM chunk_size > PAGE

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Date:   Wed, 12 Apr 2023 16:35:27 +0200
From:   Kal Cutter Conley <kal.conley@...tris.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn@...nel.org>,
        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 v5 2/4] xsk: Support UMEM chunk_size > PAGE_SIZE

> > -       pool->unaligned = unaligned;
> >         pool->frame_len = umem->chunk_size - umem->headroom -
> >                 XDP_PACKET_HEADROOM;
> > +       pool->unaligned = unaligned;
>
> nit: This change is not necessary.

Do you mind if we keep it? It makes the assignments better match the
order in the struct declaration.

> > -static void xp_check_dma_contiguity(struct xsk_dma_map *dma_map)
> > +static void xp_check_dma_contiguity(struct xsk_dma_map *dma_map, u32 page_size)
> >  {
> >         u32 i;
> >
> > -       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])
> > +       for (i = 0; i + 1 < dma_map->dma_pages_cnt; i++) {
>
> I think the previous version is clearer than this new one.

I like using `i + 1` since it matches the subscript usage. I'm used to
writing it like this for SIMD code where subtraction may wrap if the
length is unsigned, that doesn't matter in this case though. I can
restore the old way if you want.