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Subject[PATCH v3 1/5] x86: make dma_alloc_coherent() return zeroed memory if CMA is enabled
Calling dma_alloc_coherent() with __GFP_ZERO must return zeroed memory.

But when the contiguous memory allocator (CMA) is enabled on x86 and
the memory region is allocated by dma_alloc_from_contiguous(), it
doesn't return zeroed memory. Because dma_generic_alloc_coherent()
forgot to fill the memory region with zero if it was allocated by

Most implementations of dma_alloc_coherent() return zeroed memory
regardless of whether __GFP_ZERO is specified. So this fixes it by
unconditionally zeroing the allocated memory region.

Alternatively, we could fix dma_alloc_from_contiguous() to return
zeroed out memory and remove memset() from all caller of it. But we
can't simply remove the memset on arm because __dma_clear_buffer() is
used there for ensuring cache flushing and it is used in many places.
Of course we can do redundant memset in dma_alloc_from_contiguous(),
but I think this patch is less impact for fixing this problem.

Cc: Marek Szyprowski <>
Cc: Konrad Rzeszutek Wilk <>
Cc: David Woodhouse <>
Cc: Don Dutile <>
Cc: Thomas Gleixner <>
Cc: Ingo Molnar <>
Cc: "H. Peter Anvin" <>
Cc: Andi Kleen <>
Signed-off-by: Akinobu Mita <>
* Change from v2
- update commit log to describe a possible alternative fix

arch/x86/kernel/pci-dma.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
index f7d0672..a0ffe44 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -97,7 +97,6 @@ void *dma_generic_alloc_coherent(struct device *dev, size_t size,

dma_mask = dma_alloc_coherent_mask(dev, flag);

- flag |= __GFP_ZERO;
page = NULL;
/* CMA can be used only in the context which permits sleeping */
@@ -120,7 +119,7 @@ again:

return NULL;
+ memset(page_address(page), 0, size);
*dma_addr = addr;
return page_address(page);

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