netdev - Re: [net-next PATCH 1/2] mm: add dma_addr

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Date:   Mon, 11 Feb 2019 08:55:51 -0800
From:   Matthew Wilcox <willy@...radead.org>
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>,
        Saeed Mahameed <saeedm@...lanox.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 1/2] mm: add dma_addr_t to struct page

On Mon, Feb 11, 2019 at 05:06:46PM +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>

Reviewed-by: Matthew Wilcox <willy@...radead.org>

> +		struct {	/* page_pool used by netstack */
> +			/**
> +			 * @dma_addr: Page_pool need to store DMA-addr, and

s/need/needs/

> +			 * cannot use @private, as DMA-mappings can be 64-bit

s/DMA-mappings/DMA addresses/

> +			 * even on 32-bit Architectures.

s/A/a/

> +			 */
> +			dma_addr_t dma_addr; /* Shares area with @lru */

It also shares with @slab_list, @next, @compound_head, @pgmap and
@rcu_head.  I think it's pointless to try to document which other fields
something shares space with; the places which do it are a legacy from
before I rearranged struct page last year.  Anyone looking at this should
now be able to see "Oh, this is a union, only use the fields which are
in the union for the type of struct page I have here".

Are the pages allocated from this API ever supposed to be mapped to
userspace?

You also say in the documentation:

 * If no DMA mapping is done, then it can act as shim-layer that
 * fall-through to alloc_page.  As no state is kept on the page, the
 * regular put_page() call is sufficient.

I think this is probably a dangerous precedent to set.  Better to require
exactly one call to page_pool_put_page() (with the understanding that the
refcount may be elevated, so this may not be the final free of the page,
but the page will no longer be usable for its page_pool purpose).