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

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Date:	Wed, 9 Dec 2015 11:36:35 +0000
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Qais Yousef <qais.yousef@...tec.com>, linux-mips@...ux-mips.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	ralf@...ux-mips.org
Subject: Re: [PATCH] MIPS: Fix DMA contiguous allocation

On Tue, Dec 08, 2015 at 02:19:39PM -0800, Andrew Morton wrote:
> 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?

Yes, this is the correct way it should be checked. The full flags cover
watermark and kswapd treatment which potentially could be altered by
the caller.

-- 
Mel Gorman
SUSE Labs
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