Re: [PATCH net-next v7 2/2] page_pool: Track DMA-mapped pages and unmap them when destroying the pool

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexander Lobakin <aleksander.lobakin@intel.com> writes:

> From: Alexander Lobakin <aleksander.lobakin@intel.com>
> Date: Fri, 4 Apr 2025 17:55:43 +0200
>
>> From: Toke Høiland-Jørgensen <toke@redhat.com>
>> Date: Fri, 04 Apr 2025 12:18:36 +0200
>> 
>>> 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.
>> 
>> [...]
>> 
>>> -#define PP_MAGIC_MASK ~0x3UL
>>> +#define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
>>>  
>>>  /**
>>>   * struct page_pool_params - page pool parameters
>>> @@ -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 */
>> 
>> Yunsheng said this change to a full bool is redundant in the v6 thread
>> ¯\_(ツ)_/¯

AFAIU, the comment was that the second READ_ONCE() when reading the
field was redundant, because of the rcu_read_lock(). Which may be the
case, but I think keeping it makes the intent of the code clearer. And
in any case, it has nothing to do with changing the type of the field...

-Toke