[PATCH] iommu/dma: Remove broken huge page handling

[PATCH] iommu/dma: Remove broken huge page handling

[Robin Murphy]

The attempt to handle huge page allocations was originally added since
the comments around stripping __GFP_COMP in other implementations were
nonsensical, and we naively assumed that split_huge_page() could simply
be called equivalently to split_page(). It turns out that this doesn't
actually work correctly, so just get rid of it - there's little point
going to the effort of allocating huge pages if we're only going to
split them anyway.

Reported-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 drivers/iommu/dma-iommu.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index 4959f5df21bd..9194088b088f 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -524,6 +524,9 @@ 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;
@@ -544,15 +547,9 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
 			page = alloc_pages_node(nid, alloc_flags, order);
 			if (!page)
 				continue;
-			if (!order)
-				break;
-			if (!PageCompound(page)) {
+			if (order)
 				split_page(page, order);
-				break;
-			} else if (!split_huge_page(page)) {
-				break;
-			}
-			__free_pages(page, order);
+			break;
 		}
 		if (!page) {
 			__iommu_dma_free_pages(pages, i);
-- 
2.28.0.dirty

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Re: [PATCH] iommu/dma: Remove broken huge page handling

[Christoph Hellwig]

On Thu, Sep 03, 2020 at 12:34:04PM +0100, Robin Murphy wrote:
> The attempt to handle huge page allocations was originally added since
> the comments around stripping __GFP_COMP in other implementations were
> nonsensical, and we naively assumed that split_huge_page() could simply
> be called equivalently to split_page(). It turns out that this doesn't
> actually work correctly, so just get rid of it - there's little point
> going to the effort of allocating huge pages if we're only going to
> split them anyway.
> 
> Reported-by: Roman Gushchin <guro@fb.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

This looks sensibe.  We really need to clear it in generic code, but
last time I checked there were one or two buggy drivers that assumed
__GFP_COMP works and actually gives a compound page (iirc legacy
drm stuff).  All that stuff really needs fixing..

Reviewed-by: Christoph Hellwig <hch@lst.de>
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Re: [PATCH] iommu/dma: Remove broken huge page handling

[Joerg Roedel]

On Thu, Sep 03, 2020 at 12:34:04PM +0100, Robin Murphy wrote:
> The attempt to handle huge page allocations was originally added since
> the comments around stripping __GFP_COMP in other implementations were
> nonsensical, and we naively assumed that split_huge_page() could simply
> be called equivalently to split_page(). It turns out that this doesn't
> actually work correctly, so just get rid of it - there's little point
> going to the effort of allocating huge pages if we're only going to
> split them anyway.
> 
> Reported-by: Roman Gushchin <guro@fb.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/iommu/dma-iommu.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)

Applied, thanks.

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