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: <6d1e3c98-e28e-4d30-bff5-1dada745f722@gmail.com>
Date: Tue, 1 Apr 2025 10:51:46 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: 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>, Jesper Dangaard Brouer <hawk@...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 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.

-- 
Pavel Begunkov