netdev - Re: [net-next PATCH V3 1/3] mm: add dma_addr

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Message-ID: <20200429003843.rh2pasek7v5o3h63@box>
Date:   Wed, 29 Apr 2020 03:38:43 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, linux-mm@...ck.org,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        willy@...radead.org, Saeed Mahameed <saeedm@...lanox.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        mgorman@...hsingularity.net,
        "David S. Miller" <davem@...emloft.net>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [net-next PATCH V3 1/3] mm: add dma_addr_t to struct page

On Wed, Feb 13, 2019 at 02:55:40AM +0100, Jesper Dangaard Brouer wrote:
> The page_pool API is using page->private to store DMA addresses.
> As pointed out by David Miller we can't use that on 32-bit architectures
> with 64-bit DMA
> 
> This patch adds a new dma_addr_t struct to allow storing DMA addresses
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> Acked-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  include/linux/mm_types.h |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 2c471a2c43fa..0a36a22228e7 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -95,6 +95,13 @@ struct page {
>  			 */
>  			unsigned long private;
>  		};
> +		struct {	/* page_pool used by netstack */
> +			/**
> +			 * @dma_addr: might require a 64-bit value even on
> +			 * 32-bit architectures.
> +			 */
> +			dma_addr_t dma_addr;
> +		};

[ I'm slow, but I've just noticed this change into struct page. ]

Is there a change that the dma_addr would have bit 0 set? If yes it may
lead to false-positive PageTail() and really strange behaviour.

I think it's better to put some padding into the struct to avoid aliasing
to compound_head.

See commit 1d798ca3f164 ("mm: make compound_head() robust") for context.

>  		struct {	/* slab, slob and slub */
>  			union {
>  				struct list_head slab_list;	/* uses lru */
> 

-- 
 Kirill A. Shutemov