From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-id: <52DCF98B.8000900@samsung.com> Date: Mon, 20 Jan 2014 11:25:15 +0100 From: Marek Szyprowski MIME-version: 1.0 To: Andy Whitcroft Cc: LKML , x86@kernel.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , stable@vger.kernel.org Subject: Re: [PATCH] x86: dma-mapping: fix GFP_ATOMIC macro usage References: <1389948399-26344-1-git-send-email-m.szyprowski@samsung.com> In-reply-to: Content-type: text/plain; charset=UTF-8; format=flowed Content-transfer-encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: Hello, On 2014-01-17 11:49, Andy Whitcroft wrote: > On Fri, Jan 17, 2014 at 8:46 AM, Marek Szyprowski > 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@vger.kernel.org > > Signed-off-by: Marek Szyprowski > > --- > > 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