Re: [PATCH net-next v2 2/4] page_pool: add interface to manipulate frag count in page pool

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/8/10 22:58, Jesper Dangaard Brouer wrote:
> 
> 
> On 06/08/2021 04.46, Yunsheng Lin wrote:
>> For 32 bit systems with 64 bit dma, dma_addr[1] is used to
>> store the upper 32 bit dma addr, those system should be rare
>> those days.
>>
>> For normal system, the dma_addr[1] in 'struct page' is not
>> used, so we can reuse dma_addr[1] for storing frag count,
>> which means how many frags this page might be splited to.
>>
>> In order to simplify the page frag support in the page pool,
>> the PAGE_POOL_DMA_USE_PP_FRAG_COUNT macro is added to indicate
>> the 32 bit systems with 64 bit dma, and the page frag support
>> in page pool is disabled for such system.
>>
>> The newly added page_pool_set_frag_count() is called to reserve
>> the maximum frag count before any page frag is passed to the
>> user. The page_pool_atomic_sub_frag_count_return() is called
>> when user is done with the page frag.
>>
>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>> ---
>>   include/linux/mm_types.h | 18 +++++++++++++-----
>>   include/net/page_pool.h  | 46 +++++++++++++++++++++++++++++++++++++++-------
>>   net/core/page_pool.c     |  4 ++++
>>   3 files changed, 56 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 52bbd2b..7f8ee09 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -103,11 +103,19 @@ struct page {
>>               unsigned long pp_magic;
>>               struct page_pool *pp;
>>               unsigned long _pp_mapping_pad;
>> -            /**
>> -             * @dma_addr: might require a 64-bit value on
>> -             * 32-bit architectures.
>> -             */
>> -            unsigned long dma_addr[2];
>> +            unsigned long dma_addr;
>> +            union {
>> +                /**
>> +                 * dma_addr_upper: might require a 64-bit
>> +                 * value on 32-bit architectures.
>> +                 */
>> +                unsigned long dma_addr_upper;
>> +                /**
>> +                 * For frag page support, not supported in
>> +                 * 32-bit architectures with 64-bit DMA.
>> +                 */
>> +                atomic_long_t pp_frag_count;
>> +            };
>>           };
>>           struct {    /* slab, slob and slub */
>>               union {
>> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
>> index 8d7744d..42e6997 100644
>> --- a/include/net/page_pool.h
>> +++ b/include/net/page_pool.h
>> @@ -45,7 +45,10 @@
>>                       * Please note DMA-sync-for-CPU is still
>>                       * device driver responsibility
>>                       */
>> -#define PP_FLAG_ALL        (PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV)
>> +#define PP_FLAG_PAGE_FRAG    BIT(2) /* for page frag feature */
>> +#define PP_FLAG_ALL        (PP_FLAG_DMA_MAP |\
>> +                 PP_FLAG_DMA_SYNC_DEV |\
>> +                 PP_FLAG_PAGE_FRAG)
>>     /*
>>    * Fast allocation side cache array/stack
>> @@ -198,19 +201,48 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>>       page_pool_put_full_page(pool, page, true);
>>   }
>>   +#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT    \
>> +        (sizeof(dma_addr_t) > sizeof(unsigned long))
>> +
>>   static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>>   {
>> -    dma_addr_t ret = page->dma_addr[0];
>> -    if (sizeof(dma_addr_t) > sizeof(unsigned long))
>> -        ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
>> +    dma_addr_t ret = page->dma_addr;
>> +
>> +    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
>> +        ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
> 
> I find the macro name confusing.
> 
> I think it would be easier to read the code, if it was called:
>  PAGE_POOL_DMA_CANNOT_USE_PP_FRAG_COUNT

Actually, there is a *DMA* in tha above macro, which means DMA
addr uses the PP_FRAG_COUNT field.
Perhaps PAGE_POOL_DMA_ADDR_UPPER_USE_PP_FRAG_COUNT is more obvious
here?

> 
>> +
>>       return ret;
>>   }
>>     static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>>   {
>> -    page->dma_addr[0] = addr;
>> -    if (sizeof(dma_addr_t) > sizeof(unsigned long))
>> -        page->dma_addr[1] = upper_32_bits(addr);
>> +    page->dma_addr = addr;
>> +    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
>> +        page->dma_addr_upper = upper_32_bits(addr);
>> +}
>> +
>> +static inline void page_pool_set_frag_count(struct page *page, long nr)
>> +{
>> +    atomic_long_set(&page->pp_frag_count, nr);
>> +}
>> +
>> +static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
>> +                              long nr)
>> +{
>> +    long ret;
>> +
>> +    /* As suggested by Alexander, atomic_long_read() may cover up the
>> +     * reference count errors, so avoid calling atomic_long_read() in
>> +     * the cases of freeing or draining the page_frags, where we would
>> +     * not expect it to match or that are slowpath anyway.
>> +     */
>> +    if (__builtin_constant_p(nr) &&
>> +        atomic_long_read(&page->pp_frag_count) == nr)
>> +        return 0;
>> +
>> +    ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>> +    WARN_ON(ret < 0);
>> +    return ret;
>>   }
>>     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..68fab94 100644
>> --- a/net/core/page_pool.c
>> +++ b/net/core/page_pool.c
>> @@ -67,6 +67,10 @@ static int page_pool_init(struct page_pool *pool,
>>            */
>>       }
>>   +    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
>> +        pool->p.flags & PP_FLAG_PAGE_FRAG)
>> +        return -EINVAL;
> 
> I read this as: if the page_pool use pp_frag_count and have flag set, then it is invalid/no-allowed, which seems wrong.
> 
> I find this code more intuitive to read:
> 
>  +    if (PAGE_POOL_DMA_CANNOT_USE_PP_FRAG_COUNT &&
>  +        pool->p.flags & PP_FLAG_PAGE_FRAG)
>  +        return -EINVAL;
> 
> --Jesper
> 
> .
>