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

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

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

> +
>   	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