linux-kernel - Re: [PATCH] x86: dma-mapping: fix GFP

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Date:	Mon, 20 Jan 2014 11:25:15 +0100
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	Andy Whitcroft <apw@...onical.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86: dma-mapping: fix GFP_ATOMIC macro usage

Hello,

On 2014-01-17 11:49, Andy Whitcroft wrote:
> On Fri, Jan 17, 2014 at 8:46 AM, Marek Szyprowski
> <m.szyprowski@...sung.com> wrote:
> > GFP_ATOMIC is not a single gfp flag, but a macro which expands to the other
> > flags and LACK of __GFP_WAIT flag. To check if caller wanted to perform an
> > atomic allocation, the code must test __GFP_WAIT flag presence. This patch
> > fixes the issue introduced in v3.5-rc1
> >
> > CC: stable@...r.kernel.org
> > Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> > ---
> >  arch/x86/kernel/pci-dma.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> > index 872079a..32a81c9 100644
> > --- a/arch/x86/kernel/pci-dma.c
> > +++ b/arch/x86/kernel/pci-dma.c
> > @@ -100,7 +100,7 @@ void *dma_generic_alloc_coherent(struct device *dev, size_t size,
> >         flag |= __GFP_ZERO;
> >  again:
> >         page = NULL;
> > -       if (!(flag & GFP_ATOMIC))
> > +       if (flag & __GFP_WAIT)
>
> >From that description should this not actually be:
>
>      if (!(flag & (GFP_ATOMIC|__GFP_WAIT) == GFP_ATOMIC))
>
> Else we will start using this pool for more than __GFP_HIGH allocations?
>
> That said, it is possible this is right and the intent was to allow
> __GFP_HIGH allocations (in general) to use this contigious pool, but I
> will let someone more intimate with the code comment to that.  I would
> have hoped the code would have been as below in that case:
>
>      if (!(flag & __GFP_HIGH))
>
> Either way once this is resolved a nice comment should be added to
> make it really clear:

Exactly in this case, the GFP_ATOMIC check was (incorrectly) added by me in
commit 0a2b9a6ea936 ("X86: integrate CMA with DMA-mapping subsystem"). My
intention was to use CMA only if caller use other allocation flags than
GFP_ATOMIC, because CMA cannot be used from atomic context. The pool is not
aimed for __GFP_HIGH alocations. I will add additional comment to make clear
why __GFP_WAIT flag is being checked.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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