[PATCH 7/7] dma-mapping: reject __GFP_COMP in dma_alloc_attrs

Christoph Hellwig
2022-11-13T16:37:05Z

DMA allocations can never be turned back into a page pointer, so
requesting compound pages doesn't make sense and it can't even be
supported at all by various backends.

Reject __GFP_COMP with a warning in dma_alloc_attrs, and stop clearing
the flag in the arm dma ops and dma-iommu.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
arch/arm/mm/dma-mapping.c | 17 -----------------
drivers/iommu/dma-iommu.c | 3 ---
kernel/dma/mapping.c | 8 ++++++++
3 files changed, 8 insertions(+), 20 deletions(-)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index d7909091cf977..c135f6e37a00c 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -564,14 +564,6 @@ static void *__dma_alloc(struct device *dev, size_t size, dma_addr_t *handle,
if (mask < 0xffffffffULL)
gfp |= GFP_DMA;

- /*
- * Following is a work-around (a.k.a. hack) to prevent pages
- * with __GFP_COMP being passed to split_page() which cannot
- * handle them. The real problem is that this flag probably
- * should be 0 on ARM as it is not supported on this
- * platform; see CONFIG_HUGETLBFS.
- */
- gfp &= ~(__GFP_COMP);
args.gfp = gfp;

*handle = DMA_MAPPING_ERROR;
@@ -1093,15 +1085,6 @@ static void *arm_iommu_alloc_attrs(struct device *dev, size_t size,
return __iommu_alloc_simple(dev, size, gfp, handle,
coherent_flag, attrs);

- /*
- * Following is a work-around (a.k.a. hack) to prevent pages
- * with __GFP_COMP being passed to split_page() which cannot
- * handle them. The real problem is that this flag probably
- * should be 0 on ARM as it is not supported on this
- * platform; see CONFIG_HUGETLBFS.
- */
- gfp &= ~(__GFP_COMP);
-
pages = __iommu_alloc_buffer(dev, size, gfp, attrs, coherent_flag);
if (!pages)
return NULL;
diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index 9297b741f5e80..f798c44e09033 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -744,9 +744,6 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
/* IOMMU can map any pages, so himem can also be used here */
gfp |= __GFP_NOWARN | __GFP_HIGHMEM;

- /* It makes no sense to muck about with huge pages */
- gfp &= ~__GFP_COMP;
-
while (count) {
struct page *page = NULL;
unsigned int order_size;
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 33437d6206445..c026a5a5e0466 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -498,6 +498,14 @@ void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,

WARN_ON_ONCE(!dev->coherent_dma_mask);

+ /*
+ * DMA allocations can never be turned back into a page pointer, so
+ * requesting compound pages doesn't make sense (and can't even be
+ * supported at all by various backends).
+ */
+ if (WARN_ON_ONCE(flag & __GFP_COMP))
+ return NULL;
+
if (dma_alloc_from_dev_coherent(dev, size, dma_handle, &cpu_addr))
return cpu_addr;

--
2.30.2