netdev - Re: [RFC v4 18/18] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <CAHS8izPvdKZncxARWUqUqjXgoMb2MmMy+PaYa8XCcWHCQT-CSg@mail.gmail.com>
Date: Thu, 5 Jun 2025 12:47:14 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Pavel Begunkov <asml.silence@...il.com>, 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, 
	hawk@...nel.org, akpm@...ux-foundation.org, davem@...emloft.net, 
	john.fastabend@...il.com, andrew+netdev@...n.ch, toke@...hat.com, 
	tariqt@...dia.com, edumazet@...gle.com, pabeni@...hat.com, saeedm@...dia.com, 
	leon@...nel.org, ast@...nel.org, daniel@...earbox.net, david@...hat.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz, 
	rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, horms@...nel.org, 
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org, vishal.moola@...il.com
Subject: Re: [RFC v4 18/18] page_pool: access ->pp_magic through struct
 netmem_desc in page_pool_page_is_pp()

On Thu, Jun 5, 2025 at 4:49 AM Harry Yoo <harry.yoo@...cle.com> wrote:
>
> On Thu, Jun 05, 2025 at 11:56:14AM +0100, Pavel Begunkov wrote:
> > On 6/4/25 03:52, Byungchul Park wrote:
> > > To simplify struct page, the effort to separate its own descriptor from
> > > struct page is required and the work for page pool is on going.
> > >
> > > To achieve that, all the code should avoid directly accessing page pool
> > > members of struct page.
> >
> > Just to clarify, are we leaving the corresponding struct page fields
> > for now until the final memdesc conversion is done?
>
> Yes, that's correct.
>
> > If so, it might be better to leave the access in page_pool_page_is_pp()
> > to be "page->pp_magic", so that once removed the build fails until
> > the helper is fixed up to use the page->type or so.
>
> When we truly separate netmem from struct page, we won't have 'lru' field
> in memdesc (because not all types of memory are on LRU list),
> so NETMEM_DESC_ASSERT_OFFSET(lru, pp_magic) should fail.
>
> And then page_pool_page_is_pp() should be changed to check lower bits
> of memdesc pointer to identify its type.
>

Oh boy, I'm not sure that works. We already do LSB tricks with
netmem_ref to tell what kind of ref it is. I think the LSB pointer
tricks with netmem_ref and netmem_desc may trample each other's toes.
I guess we'll cross that bridge when we get to it...

-- 
Thanks,
Mina