[PATCH 44/46] mm/cma: don't release CMA pages still in use

[David Hildenbrand (Arm)]

From: Rik van Riel <riel@xxxxxxxxxxx>

When a driver calls dma_free_contiguous() before quiescing DMA, the
page might still have a reference from the device.
put_page_testzero() there returns false, WARN fires, but the code proceeds
to free_contig_frozen_range() putting a live page onto buddy and clearing
the bitmap. A subsequent allocation can hand the same PFN to a new
owner while the original holder still references it.

A concurrent put_page() that drops the last reference between the
testzero loop and free_contig_frozen_range() can double-queue the page
via page->lru, corrupting buddy lists.

Avoid the corruption by not freeing a CMA region if any of the
pages inside are still in use.

Make it explicit in the warning that the driver allowed a leak.

While at it, use page_ref_freeze() to make the semantics clearer and
switch to pr_warn_ratelimited().

Fixes: 9bda131c6093 ("mm: cma: add cma_alloc_frozen{_compound}()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
Link: https://patch.msgid.link/20260811132045.5f114772@fangorn
[ david: use page_ref_freeze() + pr_warn_ratelimited() ]
Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
---
 mm/cma.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index a10ea37a261da..871f95c9ae9f0 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -1020,7 +1020,7 @@ bool cma_release(struct cma *cma, const struct page *pages,
 		 unsigned long count)
 {
 	struct cma_memrange *cmr;
-	unsigned long ret = 0;
+	unsigned long leaked = 0;
 	unsigned long i, pfn;
 
 	cmr = find_cma_memrange(cma, pages, count);
@@ -1029,9 +1029,13 @@ bool cma_release(struct cma *cma, const struct page *pages,
 
 	pfn = page_to_pfn(pages);
 	for (i = 0; i < count; i++, pfn++)
-		ret += !put_page_testzero(pfn_to_page(pfn));
+		leaked += !page_ref_freeze(pfn_to_page(pfn), 1);
 
-	WARN(ret, "%lu pages are still in use!\n", ret);
+	if (leaked) {
+		pr_warn_ratelimited("%lu pages are still in use, not freeing CMA region!\n",
+				   leaked);
+		return true;
+	}
 
 	__cma_release_frozen(cma, cmr, pages, count);
 

-- 
2.43.0