Re: [PATCH] ARM: DMA: Fix kzalloc flags in __dma_alloc

From: Alexandre Courbot
Date: Fri Mar 18 2016 - 05:30:30 EST

On 03/18/2016 05:05 PM, Rabin Vincent wrote:
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);

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);

Makes sense. Too bad GFP_SLAB_BUG_MASK is private, otherwise we could have used it directly.

Your comment reminded me that I have applied the same "fix" to __iommu_alloc_buffer() last year:

if (array_size <= PAGE_SIZE)
pages = kzalloc(array_size, GFP_KERNEL);

... but that one is followed by this, which exists since 2012:

pages = vzalloc(array_size);

So I guess we are not too worried about sleeping in that particular function.

Anyway, I will fix as you suggested and resend.