Date: Thu, 21 Dec 2023 13:22:27 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: Shakeel Butt <shakeelb@...gle.com>, Jakub Kicinski <kuba@...nel.org>, linux-kernel@...r.kernel.org, 
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Subject: Re: [RFC PATCH net-next v1 4/4] net: page_pool: use netmem_t instead
 of struct page in API

On Thu, Dec 21, 2023 at 3:32 AM Yunsheng Lin <linyunsheng@...wei.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 the
> > drivers via inline helpers, these are:
> >
> > - skb_frag_page(), which returns NULL if the netmem is not a page, but
> > 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())

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/20231218024024.3516870-10-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_frag()?
>

Yes, v3 of this series adds skb_add_rx_frag_netmem():

https://patchwork.kernel.org/project/netdevbpf/patch/20231220214505.2303297-4-almasrymina@google.com/

> > - Some of the page_pool APIs are exposed to the drivers as static
> > inline helpers, and if I want the page_pool to use netmem internally
> > the page_pool needs to do a netmem_to_page() in these helpers.
> >
> > The refactor is not an issue, but I was wondering if not exporting
> > netmem_to_page() was worth moving the code around. I was thinking in
> > the interim until netmem is adopted and has actual driver users we may
> > prefer to just add a comment on the netmem_to_page() helper that says
> > 'try not to use this directly and use the netmem helpers instead'.
> >
>


-- 
Thanks,
Mina

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