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From:   Kal Cutter Conley <kal.conley@dectris.com>
Date:   Tue, 4 Apr 2023 14:37:27 +0200
Message-ID: <CAHApi-mXt27N0dWW1QN5qZ6OOV9uVGxc-kuEd+SBF8hDJ2NPXA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 08/10] xsk: Support UMEM chunk_size > PAGE_SIZE
To:     Magnus Karlsson <magnus.karlsson@gmail.com>
Cc:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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> > > > > > 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.
> > >
> > > Yes. While most platforms have HPAGE_SIZE defined to a compile-time
> > > constant >= 64K (very often 2M) there are platforms (at least ia64 and
> > > powerpc) where the hugepage size is configured at boot. Specifically,
> > > in the case of Itanium (ia64), the hugepage size may be configured at
> > > boot to any valid page size > PAGE_SIZE (e.g. 8K). See:
> > > https://elixir.bootlin.com/linux/latest/source/arch/ia64/mm/hugetlbpage.c#L159
> >
> > So for all practical purposes it is max 64K. Let us just write that then.
>
> What about when CONFIG_HUGETLB_PAGE is not defined? Should we keep it
> set to PAGE_SIZE in that case, or would you like it to be a fixed
> constant == 64K always?

Sorry. Now it's not clear to me if you are suggesting the
documentation be changed or the code or both?