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

[Lorenzo Stoakes (ARM)]

On Tue, Oct 27, 2026 at 12:18:04AM +0100, David Hildenbrand (Arm) wrote:
> 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>

Acked-by: Lorenzo Stoakes (ARM) <ljs@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
>

--
Cheers, Lorenzo