netdev - Re: [PATCH RFC net-next 1/3] net: page_pool: refactor dma_map into own function page_pool_dma

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Message-ID: <YDay7cWyHdJluEIc@apalos.home>
Date:   Wed, 24 Feb 2021 22:11:25 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        chuck.lever@...cle.com, netdev@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next 1/3] net: page_pool: refactor dma_map into
 own function page_pool_dma_map

On Wed, Feb 24, 2021 at 07:56:41PM +0100, Jesper Dangaard Brouer wrote:
> In preparation for next patch, move the dma mapping into its own
> function, as this will make it easier to follow the changes.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>  net/core/page_pool.c |   49 +++++++++++++++++++++++++++++--------------------
>  1 file changed, 29 insertions(+), 20 deletions(-)
> 
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index ad8b0707af04..50d52aa6fbeb 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -180,6 +180,31 @@ static void page_pool_dma_sync_for_device(struct page_pool *pool,
>  					 pool->p.dma_dir);
>  }
>  
> +static struct page * page_pool_dma_map(struct page_pool *pool,
> +				       struct page *page)
> +{

Why return a struct page* ?
boolean maybe?

> +	dma_addr_t dma;
> +
> +	/* Setup DMA mapping: use 'struct page' area for storing DMA-addr
> +	 * since dma_addr_t can be either 32 or 64 bits and does not always fit
> +	 * into page private data (i.e 32bit cpu with 64bit DMA caps)
> +	 * This mapping is kept for lifetime of page, until leaving pool.
> +	 */
> +	dma = dma_map_page_attrs(pool->p.dev, page, 0,
> +				 (PAGE_SIZE << pool->p.order),
> +				 pool->p.dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
> +	if (dma_mapping_error(pool->p.dev, dma)) {
> +		put_page(page);

This is a bit confusing when reading it. 
The name of the function should try to map the page and report a yes/no,
instead of trying to call put_page as well.
Can't we explicitly ask the user to call put_page() if the mapping failed?

A clear example is on patch 2/3, when on the first read I was convinced there
was a memory leak.

> +		return NULL;
> +	}
> +	page->dma_addr = dma;
> +
> +	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> +		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
> +
> +	return page;
> +}
> +
>  /* slow path */
>  noinline
>  static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
> @@ -187,7 +212,6 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
>  {
>  	struct page *page;
>  	gfp_t gfp = _gfp;
> -	dma_addr_t dma;
>  
>  	/* We could always set __GFP_COMP, and avoid this branch, as
>  	 * prep_new_page() can handle order-0 with __GFP_COMP.
> @@ -211,27 +235,12 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
>  	if (!page)
>  		return NULL;
>  
> -	if (!(pool->p.flags & PP_FLAG_DMA_MAP))
> -		goto skip_dma_map;
> -
> -	/* Setup DMA mapping: use 'struct page' area for storing DMA-addr
> -	 * since dma_addr_t can be either 32 or 64 bits and does not always fit
> -	 * into page private data (i.e 32bit cpu with 64bit DMA caps)
> -	 * This mapping is kept for lifetime of page, until leaving pool.
> -	 */
> -	dma = dma_map_page_attrs(pool->p.dev, page, 0,
> -				 (PAGE_SIZE << pool->p.order),
> -				 pool->p.dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
> -	if (dma_mapping_error(pool->p.dev, dma)) {
> -		put_page(page);
> -		return NULL;
> +	if (pool->p.flags & PP_FLAG_DMA_MAP) {
> +		page = page_pool_dma_map(pool, page);
> +		if (!page)
> +			return NULL;
>  	}
> -	page->dma_addr = dma;
> -
> -	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> -		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
>  
> -skip_dma_map:
>  	/* Track how many pages are held 'in-flight' */
>  	pool->pages_state_hold_cnt++;
>  
> 
> 

Thanks!
/Ilias