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

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Yunsheng Lin <linyunsheng@huawei.com>
Date: Tue, 1 Apr 2025 17:24:43 +0800

> On 2025/4/1 1:27, Zhu Yanjun wrote:
>> 在 2025/3/31 18:35, Alexander Lobakin 写道:
>>> From: Toke Høiland-Jørgensen <toke@redhat.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?
>>
>> Perhaps test_bit series functions can test the bit atomically. Maybe there are more good options about this testing the bit atomically. But test_bit should implement the task that tests the bit atomically.
> 
> There are two reading of dma_sync in this patch, the first reading is not
> under rcu read lock and doing the reading without READ_ONCE(), the second
> reading is under rcu read lock and do the reading with READ_ONCE().
> 
> The first one seems an optimization to avoid taking the rcu read lock,
> why might need READ_ONCE() to make KCSAN happy if we do care about making
> KCSAN happy.
> 
> The second one does not seems to need the atomicity by using the READ_ONCE()
> as it is always under RCU read lock(implicit or explicit one), and there is
> a rcu sync after the clearing of that bit.

IOW, are you saying this change is not needed at all?

Thanks,
Olek