linux-kernel - Re: [RFC mm v5 1/2] page_pool: check nmdesc->pp to see its usage as page pool for net

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Message-ID: <20251106180810.6b06f71a@kernel.org>
Date: Thu, 6 Nov 2025 18:08:10 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Byungchul Park <byungchul@...com>
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Subject: Re: [RFC mm v5 1/2] page_pool: check nmdesc->pp to see its usage as
 page pool for net_iov not page-backed

On Fri, 7 Nov 2025 10:59:02 +0900 Byungchul Park wrote:
> > > page-backed, the identification cannot be based on the page_type.
> > > Instead, nmdesc->pp can be used to see if it belongs to a page pool, by
> > > making sure nmdesc->pp is NULL otherwise.  
> > 
> > Please explain why. Isn't the type just a value in a field?
> > Which net_iov could also set accordingly.. ?  
> 
> page_type field is in 'struct page', so 'struct page' can check the type.
> 
> However, the field is not in 'struct net_iov', so 'struct net_iov' that
> is not backed by page, cannot use the type checking to see if it's page
> pool'ed instance.
> 
> I'm afraid I didn't get your questions.  I will try to explain again
> properly if you give me more detail and example about your questions or
> requirement.

net_iov has members in the same place as page. page_type is just 
a field right now.

static __always_inline int Page##uname(const struct page *page)		\
{									\
	return data_race(page->page_type >> 24) == PGTY_##lname;	\
}									\

The whole thing works right now by overlaying one struct on top of
another, and shared members being in the same places.

Is this clear enough?