netdev - 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: <20251108022458.GA65163@system.software.com>
Date: Sat, 8 Nov 2025 11:24:58 +0900
From: Byungchul Park <byungchul@...com>
To: Jakub Kicinski <kuba@...nel.org>
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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, Nov 07, 2025 at 05:41:29PM -0800, Jakub Kicinski wrote:
> On Fri, 7 Nov 2025 13:47:08 +0900 Byungchul Park wrote:
> > The offset of page_type in struct page cannot be used in struct net_iov
> > for the same purpose, since the offset in struct net_iov is for storing
> > (struct net_iov_area *)owner.
> 
> owner does not have to be at a fixed offset. Can we not move owner
> to _pp_mapping_pad ? Or reorder it with type, enum net_iov_type
> only has 2 values we can smoosh it with page_type easily.

I'm still confused.  I think you probably understand what this work is
for.  (I've explained several times with related links.)  Or am I
missing something from your questions?

I've answered your question directly since you asked, but the point is
that, struct net_iov will no longer overlay on struct page.

Instead, struct netmem_desc will be responsible for keeping the pp
fields while struct page will lay down the resonsibility, once the pp
fields will be removed from struct page like:

<before> (the current form is:)

   struct page {
 	memdesc_flags_t flags;
 	union {
 		...
 		struct {
 			unsigned long pp_magic;
 			struct page_pool *pp;
 			unsigned long _pp_mapping_pad;
 			unsigned long dma_addr;
 			atomic_long_t pp_ref_count;
 		};
 		...
 	};
 	unsigned int page_type;
 	...
   };
 
   struct net_iov {
 	union {
 		struct netmem_desc desc;
 		struct
 		{
 			unsigned long _flags;
 			unsigned long pp_magic;
 			struct page_pool *pp;
 			unsigned long _pp_mapping_pad;
 			unsigned long dma_addr;
 			atomic_long_t pp_ref_count;
 		};
 	};
 	struct net_iov_area *owner;
 	enum net_iov_type type;
   };

<after> (the final form should be, just around the corner:)

   struct page {
 	memdesc_flags_t flags;
 	union {
 		...
		/* pp fields are gone. */
 		...
 	};
 	unsigned int page_type;
 	...
   };
 
   struct net_iov {
 	struct netmem_desc desc;
 	struct net_iov_area *owner;
 	enum net_iov_type type;
   };

After that, struct page and struct net_iov(or struct netmem_desc) will
not share any fields with each other, probably they will be connected
e.i. through some ways between struct page and netmem_desc tho.

	Byungchul
 
> > Yeah, you can tell 'why don't we add the field, page_type, to struct
> > net_iov (or struct netmem_desc)' so as to be like:
> >
> >   struct net_iov {
> >       union {
> >               struct netmem_desc desc;
> >               struct
> >               {
> >                       unsigned long _flags;
> >                       unsigned long pp_magic;
> >                       struct page_pool *pp;
> >                       unsigned long _pp_mapping_pad;
> >                       unsigned long dma_addr;
> >                       atomic_long_t pp_ref_count;
> > +                     unsigned int page_type; // add this field newly
> >               };
> >       };
> >       struct net_iov_area *owner; // the same offet of page_type
> >       enum net_iov_type type;
> >   };
> >
> > I think we can make it anyway but it makes less sense to add page_type
> > to struct net_iov, only for PGTY_netpp.
> >
> > It'd be better to use netmem_desc->pp for that purpose, IMO.