Date: Fri, 18 Mar 2016 09:05:52 +0100
From: Rabin Vincent <rabin@rab.in>
To: Alexandre Courbot <acourbot@nvidia.com>
Cc: Russell King <linux@arm.linux.org.uk>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        gnurou@gmail.com
Subject: Re: [PATCH] ARM: DMA: Fix kzalloc flags in __dma_alloc
Message-ID: <20160318080552.GA28076@lnxrabinv.se.axis.com>
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On Fri, Mar 18, 2016 at 11:12:26AM +0900, Alexandre Courbot wrote:
> Commit 19e6e5e5392b ("ARM: 8547/1: dma-mapping: store buffer
> information") allocates a structure meant for internal buffer management
> with the GFP flags of the buffer itself. This can trigger the following
> safeguard in the slab/slub allocator:
> 
> 	if (unlikely(flags & GFP_SLAB_BUG_MASK)) {
> 		pr_emerg("gfp: %u\n", flags & GFP_SLAB_BUG_MASK);
> 		BUG();
> 	}
> 
> Fix this by allocating the structure with GFP_KERNEL, as it is meant to
> be used by the kernel and not for DMA.

We can't use GFP_KERNEL here.  The caller may have passed in gfp flags
which indicate that we can't sleep, and we need to respect that.  What we can
do is mask out the region specifiers in the gfp flags that we pass to
kzalloc().  This is what the other architectures do in their dma
allocation functions:

arch/mips/cavium-octeon/dma-octeon.c:   gfp &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
arch/mips/loongson64/common/dma-swiotlb.c:      gfp &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
arch/mips/mm/dma-default.c:     gfp &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
arch/mips/netlogic/common/nlm-dma.c:    gfp &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
arch/x86/kernel/pci-dma.c:      *gfp &= ~(__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32);