Re: [RFC PATCH net-next] page_pool: Track DMA-mapped pages and unmap them when destroying the pool

[Matthew Wilcox <willy@infradead.org>]

On Mon, Mar 10, 2025 at 10:13:32AM +0100, Toke Høiland-Jørgensen wrote:
> Yunsheng Lin <yunshenglin0825@gmail.com> writes:
> > Also, Using the more space in 'struct page' for the page_pool seems to
> > make page_pool more coupled to the mm subsystem, which seems to not
> > align with the folios work that is trying to decouple non-mm subsystem
> > from the mm subsystem by avoid other subsystem using more of the 'struct
> > page' as metadata from the long term point of view.
> 
> This seems a bit theoretical; any future changes of struct page would
> have to shuffle things around so we still have the ID available,
> obviously :)

See https://kernelnewbies.org/MatthewWilcox/Memdescs
and more immediately
https://kernelnewbies.org/MatthewWilcox/Memdescs/Path

pagepool is going to be renamed "bump" because it's a bump allocator and
"pagepool" is a nonsense name.  I haven't looked into it in a lot of
detail yet, but in the not-too-distant future, struct page will look
like this (from your point of view):

struct page {
	unsigned long flags;
	unsigned long memdesc;
	int _refcount;	// 0 for bump
	union {
		unsigned long private;
		atomic_t _mapcount; // maybe used by bump?  not sure
	};
};

'memdesc' will be a pointer to struct bump with the bottom four bits of
that pointer indicating that it's a struct bump pointer (and not, say, a
folio or a slab).

So if you allocate a multi-page bump, you'll get N of these pages,
and they'll all point to the same struct bump where you'll maintain
your actual refcount.  And you'll be able to grow struct bump to your
heart's content.  I don't know exactly what struct bump looks like,
but the core mm will have no requirements on you.