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: <ff346763-07ad-4323-a46c-974adc71c121@gmail.com>
Date: Wed, 12 Mar 2025 12:55:59 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Matthew Wilcox <willy@...radead.org>,
 Toke Høiland-Jørgensen <toke@...hat.com>
Cc: 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 3/11/25 16:46, Matthew Wilcox wrote:
> 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.

Which is why it's so great we have you here, not every one is
developing xarray. So maybe it is useless for this case then.

-- 
Pavel Begunkov