linux-kernel - Re: [PATCH 18/18] mm, netmem: remove the page pool members in struct page

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Message-ID: <CAHS8izM5xd=cBRuUpAUNtcFzZ3hMwmseyh6rsV+WPRAvdzv4cA@mail.gmail.com>
Date: Mon, 26 May 2025 10:33:52 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Byungchul Park <byungchul@...com>, willy@...radead.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, kernel_team@...ynix.com, 
	kuba@...nel.org, ilias.apalodimas@...aro.org, harry.yoo@...cle.com, 
	hawk@...nel.org, akpm@...ux-foundation.org, davem@...emloft.net, 
	john.fastabend@...il.com, andrew+netdev@...n.ch, toke@...hat.com, 
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	leon@...nel.org, ast@...nel.org, daniel@...earbox.net, david@...hat.com, 
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	linux-rdma@...r.kernel.org, bpf@...r.kernel.org, vishal.moola@...il.com
Subject: Re: [PATCH 18/18] mm, netmem: remove the page pool members in struct page

On Mon, May 26, 2025 at 9:57 AM Pavel Begunkov <asml.silence@...il.com> wrote:
> >> Removing these asserts is actually a bit dangerous. Functions like
> >> netmem_or_pp_magic() rely on the fact that the offsets are the same
> >> between struct page and struct net_iov to access these fields without
> >
> > Worth noting this patch removes the page pool fields from struct page.
>
> static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> {
>         return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
> }
>
> static inline atomic_long_t *netmem_get_pp_ref_count_ref(netmem_ref netmem)
> {
>         return &__netmem_clear_lsb(netmem)->pp_ref_count;
> }
>
> That's a snippet of code after applying the series. So, let's say we
> take a page, it's casted to netmem, then the netmem (as it was before)
> is casted to net_iov. Before it relied on net_iov and the pp's part of
> the page having the same layout, which was checked by static asserts,
> but now, unless I'm mistaken, it's aligned in the exactly same way but
> points to a seemingly random offset of the page. We should not be doing
> that.
>

Agreed.

> Just to be clear, I think casting pages to struct net_iov *, as it
> currently is, is quite ugly, but that's something netmem_desc and this
> effort can help with.
>

Agreed it's quite ugly. It was done in the name of optimizing the page
pool benchmark to the extreme as far as I can remember. We could use
page pool benchmark numbers on this series to make sure these new
changes aren't regressing the fast path.

-- 
Thanks,
Mina