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From: Mina Almasry <almasrymina@google.com>
Date: Tue, 2 Jan 2024 08:14:17 -0800
Message-ID: <CAHS8izNqeiK1tq=48LMbbqq5B4d2mhgbuKRvnFtiBngf73jXZg@mail.gmail.com>
Subject: Re: [RFC PATCH net-next v1 4/4] net: page_pool: use netmem_t instead
 of struct page in API
To: Yunsheng Lin <linyunsheng@huawei.com>
Cc: Shakeel Butt <shakeelb@google.com>, Jakub Kicinski <kuba@kernel.org>, linux-kernel@vger.kernel.org, 
	netdev@vger.kernel.org, bpf@vger.kernel.org, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
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	"H. Peter Anvin" <hpa@zytor.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, 
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	=?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>, 
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	Saeed Mahameed <saeedm@nvidia.com>, Leon Romanovsky <leon@kernel.org>, 
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Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Thu, Dec 21, 2023 at 10:42=E2=80=AFPM Yunsheng Lin <linyunsheng@huawei.c=
om> wrote:
>
> On 2023/12/22 5:22, Mina Almasry wrote:
> > On Thu, Dec 21, 2023 at 3:32=E2=80=AFAM Yunsheng Lin <linyunsheng@huawe=
i.com> wrote:
> >>
> >> On 2023/12/20 11:01, Mina Almasry wrote:
> >>
> >> ...
> >>
> >>>>>> Perhaps we should aim to not export netmem_to_page(),
> >>>>>> prevent modules from accessing it directly.
> >>>>>
> >>>>> +1.
> >>>>
> >>>
> >>> I looked into this, but it turns out it's a slightly bigger change
> >>> that needs some refactoring to make it work. There are few places
> >>> where I believe I need to add netmem_to_page() that are exposed to th=
e
> >>> drivers via inline helpers, these are:
> >>>
> >>> - skb_frag_page(), which returns NULL if the netmem is not a page, bu=
t
> >>> needs to do a netmem_to_page() to return the page otherwise.
> >>
> >> Is it possible to introduce something like skb_frag_netmem() for
> >> netmem? so that we can keep most existing users of skb_frag_page()
> >> unchanged and avoid adding additional checking overhead for existing
> >> users.
> >>
> >
> > In my experience most current skb_frag_page() users need specifically
> > the struct page*. Example is illegal_highdma() which
> > PageHighMem(skb_frag_page())
>
> For illegal_highdma() case, is it possible to use something like
> skb_readabe_frag() checking to avoid calling skb_frag_page() for netmem?
>

Not teh skb_readable_frag() check I think, because illegal_highdma()
is not trying to read the SKB per se.

But I agree with your general point, and that should be handled
correctly in this patch which adds devmem support:

https://patchwork.kernel.org/project/netdevbpf/patch/20231218024024.3516870=
-10-almasrymina@google.com/

The idea being that skb_frag_page() can return NULL if the frag is not
paged, and the relevant callers are modified to handle that.

> >
> > But RFC v5 adds skb_frag_netmem() for callsites that want a netmem and
> > don't care about specifically a page:
> >
> > https://patchwork.kernel.org/project/netdevbpf/patch/20231218024024.351=
6870-10-almasrymina@google.com/
> >
> >>> - The helpers inside skb_add_rx_frag(), which needs to do a
> >>> netmem_to_page() to set skb->pfmemalloc.
> >>
> >> Similar as above, perhaps introduce something like skb_add_rx_netmem_f=
rag()?
> >>
> >
> > Yes, v3 of this series adds skb_add_rx_frag_netmem():
> >
> > https://patchwork.kernel.org/project/netdevbpf/patch/20231220214505.230=
3297-4-almasrymina@google.com/


--=20
Thanks,
Mina