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

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Date:	Fri, 17 Jan 2014 10:49:43 +0000
From:	Andy Whitcroft <apw@...onical.com>
To:	Marek Szyprowski <m.szyprowski@...sung.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

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:

-apw
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