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

Leon Romanovsky
2022-11-14T08:11:59Z

On Sun, Nov 13, 2022 at 05:35:35PM +0100, Christoph Hellwig wrote:
> 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;

In RDMA patches, you wrote that GFP_USER is not legal flag either. So it
is better to WARN here for everything that is not allowed.

> +
> if (dma_alloc_from_dev_coherent(dev, size, dma_handle, &cpu_addr))
> return cpu_addr;
>
> --
> 2.30.2
>