Re: [PATCH rfc v3 2/4] page_pool: add interface for getting and setting pagecnt_bias

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/7/13 2:08, Ilias Apalodimas wrote:
> [...]
>>>> +static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>>>>   {
>>>> +       if (WARN_ON(addr & ~PAGE_MASK))
>>>> +               return false;
>>>> +
>>>>          page->dma_addr[0] = addr;
>>>>          if (sizeof(dma_addr_t) > sizeof(unsigned long))
>>>>                  page->dma_addr[1] = upper_32_bits(addr);
>>>> +
>>>> +       return true;
>>>> +}
>>>> +
>>>
>>> Rather than making this a part of the check here it might make more
>>> sense to pull this out and perform the WARN_ON after the check for
>>> dma_mapping_error.
>>
>> I need to point out that I don't like WARN_ON and BUG_ON code in fast-path
>> code, because compiler adds 'ud2' assembler instructions that influences the
>> instruction-cache fetching in the CPU.  Yes, I have seen a measuresable
>> impact from this before.
>>
>>
>>> Also it occurs to me that we only really have to do this in the case
>>> where dma_addr_t is larger than the size of a long. Otherwise we could
>>> just have the code split things so that dma_addr[0] is the dma_addr
>>> and dma_addr[1] is our pagecnt_bias value in which case we could
>>> probably just skip the check.
>>
>> The dance to get 64-bit DMA addr on 32-bit systems is rather ugly and
>> confusing, sadly.  We could take advantage of this, I just hope this will
>> not make it uglier.
> 
> Note here that we can only use this because dma_addr is not aliased to
> compound page anymore (after the initial page_pool recycling patchset). 
> We must keep this in mind if we even restructure struct page.
> 
> Can we do something more radical for this? The 64/32 bit dance is only
> there for 32 bit systems with 64 bit dma.  Since the last time we asked
> about this no one seemed to care about these, and I really doubt we'll get
> an ethernet driver for them (that needs recycling....), can we *only* support 
> frag allocation and recycling for 'normal' systems? We could always just r
> e-purpose dma_addr[1] for those.

Will define a macro for "sizeof(dma_addr_t) > sizeof(unsigned long)" to
decide whether to use the dma_addr[1], hopefully the compiler will optimize
out the unused code in a specific system.

> 
> Regards
> /Ilias
> 
>>
>>
>>>> +static inline int page_pool_get_pagecnt_bias(struct page *page)
>>>> +{
>>>> +       return READ_ONCE(page->dma_addr[0]) & ~PAGE_MASK;
>>>> +}
>>>> +
>>>> +static inline unsigned long *page_pool_pagecnt_bias_ptr(struct page *page)
>>>> +{
>>>> +       return page->dma_addr;
>>>> +}
>>>> +
>>>> +static inline void page_pool_set_pagecnt_bias(struct page *page, int bias)
>>>> +{
>>>> +       unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
>>>> +
>>>> +       dma_addr_0 &= PAGE_MASK;
>>>> +       dma_addr_0 |= bias;
>>>> +
>>>> +       WRITE_ONCE(page->dma_addr[0], dma_addr_0);
>>>>   }
>>>>
>>>>   static inline bool is_page_pool_compiled_in(void)
>>>> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
>>>> index 78838c6..1abefc6 100644
>>>> --- a/net/core/page_pool.c
>>>> +++ b/net/core/page_pool.c
>>>> @@ -198,7 +198,13 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>>>>          if (dma_mapping_error(pool->p.dev, dma))
>>>>                  return false;
>>>>
>>>
>>> So instead of adding to the function below you could just add your
>>> WARN_ON check here with the unmapping call.

Ok.

>>>
>>>> -       page_pool_set_dma_addr(page, dma);
>>>> +       if (unlikely(!page_pool_set_dma_addr(page, dma))) {
>>>> +               dma_unmap_page_attrs(pool->p.dev, dma,
>>>> +                                    PAGE_SIZE << pool->p.order,
>>>> +                                    pool->p.dma_dir,
>>>> +                                    DMA_ATTR_SKIP_CPU_SYNC);
>>>> +               return false;
>>>> +       }
>>>>
>>>>          if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
>>>>                  page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
>>>> --
>>>> 2.7.4
>>>>
>>>
>>
> .
>