linux-kernel - Re: [PATCH] MIPS: Fix DMA contiguous allocation

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Date:	Tue, 8 Dec 2015 14:19:39 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Qais Yousef <qais.yousef@...tec.com>
Cc:	<linux-mips@...ux-mips.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <ralf@...ux-mips.org>,
	<mgorman@...hsingularity.net>
Subject: Re: [PATCH] MIPS: Fix DMA contiguous allocation

On Tue, 8 Dec 2015 10:18:50 +0000 Qais Yousef <qais.yousef@...tec.com> wrote:

> Recent changes to how GFP_ATOMIC is defined seems to have broken the condition
> to use mips_alloc_from_contiguous() in mips_dma_alloc_coherent().
> 
> I couldn't bottom out the exact change but I think it's this one
> 
> d0164adc89f6 (mm, page_alloc: distinguish between being unable to sleep,
> unwilling to sleep and avoiding waking kswapd)
> 
> >From what I see GFP_ATOMIC has multiple bits set and the check for !(gfp
> & GFP_ATOMIC) isn't enough. To verify if the flag is atomic we need to make
> sure that (gfp & GFP_ATOMIC) == GFP_ATOMIC to verify that all bits rquired to
> satisfy GFP_ATOMIC condition are set.
> 
> ...
>
> --- a/arch/mips/mm/dma-default.c
> +++ b/arch/mips/mm/dma-default.c
> @@ -145,7 +145,7 @@ static void *mips_dma_alloc_coherent(struct device *dev, size_t size,
>  
>  	gfp = massage_gfp_flags(dev, gfp);
>  
> -	if (IS_ENABLED(CONFIG_DMA_CMA) && !(gfp & GFP_ATOMIC))
> +	if (IS_ENABLED(CONFIG_DMA_CMA) && ((gfp & GFP_ATOMIC) != GFP_ATOMIC))
>  		page = dma_alloc_from_contiguous(dev,
>  					count, get_order(size));
>  	if (!page)

hm.  It seems that the code is asking "can I do a potentially-sleeping
memory allocation"?

The way to do that under the new regime is

	if (IS_ENABLED(CONFIG_DMA_CMA) && gfpflags_allow_blocking(gfp))

Mel, can you please confirm?
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