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: <2cb9c1fd-db44-4f66-9c5b-03155c6187d6@gmail.com>
Date: Tue, 11 Mar 2025 13:56:29 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
 Mina Almasry <almasrymina@...gle.com>, David Wei <dw@...idwei.uk>
Cc: 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 13:44, Toke Høiland-Jørgensen wrote:
> Pavel Begunkov <asml.silence@...il.com> writes:
> 
>> On 3/9/25 12:42, Toke Høiland-Jørgensen wrote:
>>> Mina Almasry <almasrymina@...gle.com> writes:
...
>>> No, pp_magic was also my backup plan (see the other thread). Tried
>>> actually doing that now, and while there's a bit of complication due to
>>> the varying definitions of POISON_POINTER_DELTA across architectures,
>>> but it seems that this can be defined at compile time. I'll send a v2
>>> RFC with this change.
>>
>> FWIW, personally I like this one much more than an extra indirection
>> to pp.
>>
>> 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.

And I just noticed there is an entire discussion my email
client didn't pull :)

At least that's likely the easiest solution. Depends on how
complicated it is to fit the index in, but there is an option
to just go with it and continue the discussion on how to
improve it on top.

> 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.

-- 
Pavel Begunkov