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

From: Magnus Karlsson <magnus.karlsson@gmail.com>
To: Kal Cutter Conley <kal.conley@dectris.com>
Cc: "Björn Töpel" <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
Subject: Re: [PATCH bpf-next v2 08/10] xsk: Support UMEM chunk_size > PAGE_SIZE
Date: Tue, 4 Apr 2023 13:14:20 +0200	[thread overview]
Message-ID: <CAJ8uoz0GgzzfrgS0189=zwY-zzogZq+=v-NCY7O+RuWrwe1n6w@mail.gmail.com> (raw)
In-Reply-To: <CAHApi-=ui3JofMr7y+LvuYkXCU=h7vGiKXsfuV5gog-02u-u+Q@mail.gmail.com>

On Tue, 4 Apr 2023 at 12:29, Kal Cutter Conley <kal.conley@dectris.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.
>
> 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.

> >
> > > > >  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.
> >
>
> Would you be OK if I just remove the ones from xsk_buff_pool? I think
> the code in xdp_umem.c is quite readable and the #ifdefs are really
> only used in xdp_umem_pin_pages.

Please make an effort to remove the ones in xdp_umem.c too. The more
ifdefs you add, the harder it will be to read.