linux-kernel - Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

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Message-ID: <1b8e2681-ccd6-81e0-b696-8b6c26e31f26@huawei.com>
Date:   Mon, 21 Aug 2023 20:18:55 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>
CC:     Mina Almasry <almasrymina@...gle.com>, <davem@...emloft.net>,
        <pabeni@...hat.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Liang Chen <liangchen.linux@...il.com>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit
 arch with 64-bit DMA

On 2023/8/19 5:51, Jakub Kicinski wrote:
> On Fri, 18 Aug 2023 09:12:09 +0300 Ilias Apalodimas wrote:
>>> Right, IIUC we don't have enough space to fit dma_addr_t and the
>>> refcount, but if we store the dma addr on a shifted u32 instead
>>> of using dma_addr_t explicitly - the refcount should fit?  
>>
>> struct page looks like this:
>>
>> unsigned long dma_addr;
>> union {
>>       unsigned long dma_addr_upper;
>>       atomic_long_t pp_frag_count;
>> };
> 
> I could be completely misunderstanding the problem.
> Let me show you the diff of what I was thinking more or less.
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5e74ce4a28cd..58ffa8dc745f 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -126,11 +126,6 @@ struct page {
>  			unsigned long _pp_mapping_pad;
>  			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.
> diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> index 94231533a369..6f87a0fa2178 100644
> --- a/include/net/page_pool/helpers.h
> +++ b/include/net/page_pool/helpers.h
> @@ -212,16 +212,24 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  	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;
> +		ret <<= PAGE_SHIFT;
>  
>  	return ret;
>  }
>  
> -static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
> +static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>  {
> +	bool failed = false;
> +
>  	page->dma_addr = addr;
> -	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> -		page->dma_addr_upper = upper_32_bits(addr);
> +	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT) {
> +		page->dma_addr >>= PAGE_SHIFT;
> +		/* We assume page alignment to shave off bottom bits,
> +		 * if this "compression" doesn't work we need to drop.
> +		 */
> +		failed = addr != page->dma_addr << PAGE_SHIFT;
> +	}
> +	return failed;
>  }
>  
>  static inline bool page_pool_put(struct page_pool *pool)
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 77cb75e63aca..9ea42e242a89 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -211,10 +211,6 @@ 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;
> -
>  #ifdef CONFIG_PAGE_POOL_STATS
>  	pool->recycle_stats = alloc_percpu(struct page_pool_recycle_stats);
>  	if (!pool->recycle_stats)
> @@ -359,12 +355,19 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>  	if (dma_mapping_error(pool->p.dev, dma))
>  		return false;
>  
> -	page_pool_set_dma_addr(page, dma);
> +	if (page_pool_set_dma_addr(page, dma))
> +		goto unmap_failed;

What does the driver do when the above fails?
Does the driver still need to implement a fallback for 32 bit arch with
dma addr with more than 32 + 12 bits?
If yes, it does not seems to be very helpful from driver's point of view
as the driver might still need to call page allocator API directly when
the above fails.

>  
>  	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
>  		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
>  
>  	return true;
> +
> +unmap_failed:
> +	dma_unmap_page_attrs(pool->p.dev, dma,
> +			     PAGE_SIZE << pool->p.order, pool->p.dma_dir,
> +			     DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
> +	return false;
>  }
>  
>  static void page_pool_set_pp_info(struct page_pool *pool,
> .
>