[RFC PATCH v3] mm/cma: don't release CMA pages still in use

[Rik van Riel <riel@surriel.com>]

When a driver calls dma_free_contiguous() before quiescing DMA, the
page still has 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.

Fixes: 9bda131c6093 ("mm: cma: add cma_alloc_frozen{_compound}()")
Link: https://lore.kernel.org/linux-mm/20260809210608.06b5ccb9@fangorn/ [v1]
Link: https://lore.kernel.org/linux-mm/20260810122737.030f8452@fangorn/ [v2]
Cc: stable@vger.kernel.org
Signed-off-by: Rik van Riel <riel@surriel.com>
---
v3:
 - simplify things by simply leaking the CMA range, drivers should not
   call cma_release() while the space is still in use (David Hildenbrand)

 mm/cma.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index a13ce4999b39..c1425f25c39b 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -1018,7 +1018,7 @@ 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);
@@ -1027,9 +1027,12 @@ 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 += !put_page_testzero(pfn_to_page(pfn));
 
-	WARN(ret, "%lu pages are still in use!\n", ret);
+	if (leaked) {
+		WARN(1, "%lu pages are still in use, not freeing CMA region!\n", leaked);
+		return true;
+	}
 
 	__cma_release_frozen(cma, cmr, pages, count);
 
-- 
2.55.0