Re: [PATCH v2] mm/cma: move put_page_testzero() out of VM_WARN_ON in cma_release()

[SeongJae Park]

On Tue, 24 Feb 2026 21:28:52 -0500 Zi Yan <ziy@xxxxxxxxxx> 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.
> 
> 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 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..ea540d3f473c 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));

So, 'ret' will be incremented when put_page_testzero() returns 'true', right?
Shouldn't 'ret' be inceremented only when put_page_testzero() returns 'false'?

> +
> +	WARN(ret, "%lu pages are still in use!\n", ret);

This will trigger the warning even if CONFIG_DEBUG_VM is unset.  I'm fine with
the changed behavior, but I'm curious if that is the real intention of this
revision.


Thanks,
SJ

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