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: <Z9Bo9osGdjTWct98@casper.infradead.org>
Date: Tue, 11 Mar 2025 16:46:46 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Pavel Begunkov <asml.silence@...il.com>,
	Mina Almasry <almasrymina@...gle.com>, David Wei <dw@...idwei.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Ilias Apalodimas <ilias.apalodimas@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Yunsheng Lin <linyunsheng@...wei.com>,
	Yonglong Liu <liuyonglong@...wei.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 02:44:15PM +0100, Toke Høiland-Jørgensen wrote:
> Pavel Begunkov <asml.silence@...il.com> writes:
> > If we're out of space in the page, why can't we use struct page *
> > as indices into the xarray? Ala
> >
> > struct page *p = ...;
> > xa_store(xarray, index=(unsigned long)p, p);
> >
> > Indices wouldn't be nicely packed, but it's still a map. Is there
> > a problem with that I didn't consider?
> 
> Huh. As I just replied to Yunsheng, I was under the impression that this
> was not supported. But since you're now the second person to suggest
> this, I looked again, and it looks like I was wrong. There does indeed
> seem to be other places in the kernel that does this.
> 
> As you say the indices won't be as densely packed, though. So I'm
> wondering if using the bits in pp_magic would be better in any case to
> get the better packing? I guess we can try benchmarking both approaches
> and see if there's a measurable difference.

This is an absolutely terrible idea, only proposed by those who have no
understanding of how the XArray works.  It could not be more wasteful.