Re: [PATCH v3] mm/cma: move put_page_testzero() out of VM_WARN_ON in cma_release()

[Anshuman Khandual]

On 25/02/26 8:42 AM, Zi Yan wrote:
> When CONFIG_DEBUG_VM is not set, VM_WARN_ON is a NOP. Putting any statement
> with side effect inside it is incorrect. Collect all !put_page_testzero()
> results and check the sum using WARN instead after the loop. It restores
> the same check in free_contig_range() before commit
> e0c1326779cc ("mm: page_alloc: add alloc_contig_frozen_{range,pages}()"),
> the commit prior to the Fixes one.
> 
> Fixes: 9bda131c6093 ("mm: cma: add cma_alloc_frozen{_compound}()")
> Reported-by: Ron Economos <re@xxxxxxxx>
> Closes: https://lore.kernel.org/all/1b17c38f-30d3-4bb4-a7e1-e74b19ada885@xxxxxxxx/
> Suggested-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
> From V2:
> - Collect !put_page_testzero() to get the right result.
> 
> From V1:
> - Collect all put_page_testzero() results and do a single WARN after the
>   loop.
> 
>  mm/cma.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/cma.c b/mm/cma.c
> index 94b5da468a7d..15cc0ae76c8e 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -1013,6 +1013,7 @@ bool cma_release(struct cma *cma, const struct page *pages,
>  		 unsigned long count)
>  {
>  	struct cma_memrange *cmr;
> +	unsigned long ret = 0;
>  	unsigned long i, pfn;
>  
>  	cmr = find_cma_memrange(cma, pages, count);
> @@ -1021,7 +1022,9 @@ bool cma_release(struct cma *cma, const struct page *pages,
>  
>  	pfn = page_to_pfn(pages);
>  	for (i = 0; i < count; i++, pfn++)
> -		VM_WARN_ON(!put_page_testzero(pfn_to_page(pfn)));
> +		ret += !put_page_testzero(pfn_to_page(pfn));
> +
> +	WARN(ret, "%lu pages are still in use!\n", ret);
>  
>  	__cma_release_frozen(cma, cmr, pages, count);
>  

Makes sense to make the above warning unconditional regardless of CONFIG_DEBUG_VM.

Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>