netdev - Re: [PATCH net-next v5 2/2] page_pool: Track DMA-mapped pages and unmap them when destroying the pool

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Message-ID: <dcecccee-f780-4114-a957-275757f4235d@kernel.org>
Date: Tue, 1 Apr 2025 14:06:24 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Pavel Begunkov <asml.silence@...il.com>, Paolo Abeni <pabeni@...hat.com>,
 Alexander Lobakin <aleksander.lobakin@...el.com>,
 Toke Høiland-Jørgensen <toke@...hat.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski
 <kuba@...nel.org>, Saeed Mahameed <saeedm@...dia.com>,
 Leon Romanovsky <leon@...nel.org>, Tariq Toukan <tariqt@...dia.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, Eric Dumazet <edumazet@...gle.com>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 Simon Horman <horms@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 Mina Almasry <almasrymina@...gle.com>, Yonglong Liu
 <liuyonglong@...wei.com>, Yunsheng Lin <linyunsheng@...wei.com>,
 Matthew Wilcox <willy@...radead.org>, netdev@...r.kernel.org,
 bpf@...r.kernel.org, linux-rdma@...r.kernel.org, linux-mm@...ck.org,
 Qiuling Ren <qren@...hat.com>, Yuying Ma <yuma@...hat.com>
Subject: Re: [PATCH net-next v5 2/2] page_pool: Track DMA-mapped pages and
 unmap them when destroying the pool

On 01/04/2025 11.51, Pavel Begunkov wrote:
> On 4/1/25 09:56, Paolo Abeni wrote:
>> On 3/31/25 6:35 PM, Alexander Lobakin wrote:
>>> From: Toke Høiland-Jørgensen <toke@...hat.com>
>>> Date: Fri, 28 Mar 2025 13:19:09 +0100
>>>
>>>> When enabling DMA mapping in page_pool, pages are kept DMA mapped until
>>>> they are released from the pool, to avoid the overhead of re-mapping 
>>>> the
>>>> pages every time they are used. This causes resource leaks and/or
>>>> crashes when there are pages still outstanding while the device is torn
>>>> down, because page_pool will attempt an unmap through a non-existent 
>>>> DMA
>>>> device on the subsequent page return.
>>>
>>> [...]
>>>
>>>> @@ -173,10 +212,10 @@ struct page_pool {
>>>>       int cpuid;
>>>>       u32 pages_state_hold_cnt;
>>>> -    bool has_init_callback:1;    /* slow::init_callback is set */
>>>> +    bool dma_sync;            /* Perform DMA sync for device */
>>>
>>> Have you seen my comment under v3 (sorry but I missed that there was v4
>>> already)? Can't we just test the bit atomically?
>>
>> My understanding is that to make such operation really atomic, we will
>> need to access all the other bits within the same bitfield with atomic
>> bit ops, leading to a significant code churn (and possibly some 
>> overhead).
>>
>> I think that using a full bool field is a better option.
> 
> I agree, it's better not to overcomplicate a fix, and we can always
> return to it later.

I also agree. No need to do atomic bit operations.
Let's not complicate the code because we can.
I prefer keeping code readable.

--Jesper