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Date:   Sat, 10 Apr 2021 16:48:24 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Matteo Croce <mcroce@linux.microsoft.com>
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Subject: Re: [PATCH net-next v3 2/5] mm: add a signature in struct page
Message-ID: <20210410154824.GZ2531743@casper.infradead.org>
References: <20210409223801.104657-1-mcroce@linux.microsoft.com>
 <20210409223801.104657-3-mcroce@linux.microsoft.com>
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On Sat, Apr 10, 2021 at 12:37:58AM +0200, Matteo Croce wrote:
> This is needed by the page_pool to avoid recycling a page not allocated
> via page_pool.

Is the PageType mechanism more appropriate to your needs?  It wouldn't
be if you use page->_mapcount (ie mapping it to userspace).

> Signed-off-by: Matteo Croce <mcroce@microsoft.com>
> ---
>  include/linux/mm_types.h | 1 +
>  include/net/page_pool.h  | 2 ++
>  net/core/page_pool.c     | 4 ++++
>  3 files changed, 7 insertions(+)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6613b26a8894..ef2d0d5f62e4 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -101,6 +101,7 @@ struct page {
>  			 * 32-bit architectures.
>  			 */
>  			dma_addr_t dma_addr;
> +			unsigned long signature;
>  		};
>  		struct {	/* slab, slob and slub */
>  			union {
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index b5b195305346..b30405e84b5e 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -63,6 +63,8 @@
>   */
>  #define PP_ALLOC_CACHE_SIZE	128
>  #define PP_ALLOC_CACHE_REFILL	64
> +#define PP_SIGNATURE		0x20210303
> +
>  struct pp_alloc_cache {
>  	u32 count;
>  	void *cache[PP_ALLOC_CACHE_SIZE];
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index ad8b0707af04..2ae9b554ef98 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -232,6 +232,8 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
>  		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
>  
>  skip_dma_map:
> +	page->signature = PP_SIGNATURE;
> +
>  	/* Track how many pages are held 'in-flight' */
>  	pool->pages_state_hold_cnt++;
>  
> @@ -302,6 +304,8 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
>  			     DMA_ATTR_SKIP_CPU_SYNC);
>  	page->dma_addr = 0;
>  skip_dma_unmap:
> +	page->signature = 0;
> +
>  	/* This may be the last page returned, releasing the pool, so
>  	 * it is not safe to reference pool afterwards.
>  	 */
> -- 
> 2.30.2
>