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

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Message-ID: <Z9BSlzpbNRL2MzPj@casper.infradead.org>
Date: Tue, 11 Mar 2025 15:11:19 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
	Yunsheng Lin <yunshenglin0825@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Ilias Apalodimas <ilias.apalodimas@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Yonglong Liu <liuyonglong@...wei.com>,
	Mina Almasry <almasrymina@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, linux-mm@...ck.org,
	netdev@...r.kernel.org
Subject: Re: [RFC PATCH net-next] page_pool: Track DMA-mapped pages and unmap
 them when destroying the pool

On Tue, Mar 11, 2025 at 08:25:25PM +0800, Yunsheng Lin wrote:
> > struct page {
> > 	unsigned long flags;
> > 	unsigned long memdesc;
> 
> It seems there may be memory behind the above 'memdesc' with different size
> and layout for different subsystem?

Yes.

> I am not sure if I understand the case of the same page might be handle in
> two subsystems concurrently or a page is allocated in one subsystem and
> then passed to be handled in other subsystem, for examlpe:
> page_pool owned page is mmap'ed into user space through tcp zero copy,
> see tcp_zerocopy_vm_insert_batch(), it seems the same page is handled in
> both networking/page_pool and vm subsystem?

It's not that arbitrary.  I mean, you could read all the documentation
I've written about this concept, listen to the talks I've given.  But
sure, you're a special fucking snowflake and deserve your own unique
explanation.