[PATCH] dma-direct: Clear pages before coherent remap

[Rosen Penev <rosenp@gmail.com>]

Clear pages through their page mapping before creating a coherent
remap for dma-direct allocations. Some architectures implement the
coherent remap as uncached memory, where the generic memset() path may
use cache-only zeroing instructions that are not valid for the returned
CPU mapping.

Keep the existing memset() for non-remapped allocations, but avoid
normal memset() on the remapped coherent allocation path.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 kernel/dma/direct.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 583c5922bca2..76f7bf43bd28 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -8,6 +8,7 @@
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/dma-map-ops.h>
+#include <linux/highmem.h>
 #include <linux/scatterlist.h>
 #include <linux/pfn.h>
 #include <linux/vmalloc.h>
@@ -104,6 +105,15 @@ static void __dma_direct_free_pages(struct device *dev, struct page *page,
 	dma_free_contiguous(dev, page, size);
 }
 
+static void dma_direct_zero_pages(struct page *page, size_t size)
+{
+	unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
+	unsigned long i;
+
+	for (i = 0; i < count; i++)
+		clear_highpage(page + i);
+}
+
 static struct page *dma_direct_alloc_swiotlb(struct device *dev, size_t size)
 {
 	struct page *page = swiotlb_alloc(dev, size);
@@ -268,6 +278,13 @@ void *dma_direct_alloc(struct device *dev, size_t size,
 	if (remap) {
 		pgprot_t prot = dma_pgprot(dev, PAGE_KERNEL, attrs);
 
+		/*
+		 * Zero via the page mapping before creating a potentially
+		 * uncached remap.  Some architectures cannot safely run normal
+		 * memset on uncached memory.
+		 */
+		dma_direct_zero_pages(page, size);
+
 		if (force_dma_unencrypted(dev))
 			prot = pgprot_decrypted(prot);
 
@@ -283,10 +300,9 @@ void *dma_direct_alloc(struct device *dev, size_t size,
 		ret = page_address(page);
 		if (dma_set_decrypted(dev, ret, size))
 			goto out_leak_pages;
+		memset(ret, 0, size);
 	}
 
-	memset(ret, 0, size);
-
 	if (set_uncached) {
 		arch_dma_prep_coherent(page, size);
 		ret = arch_dma_set_uncached(ret, size);
-- 
2.54.0