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

[SeongJae Park <sj@kernel.org>]

On Tue, 24 Feb 2026 22:04:15 -0500 Zi Yan <ziy@nvidia.com> wrote:

> On 24 Feb 2026, at 21:54, SeongJae Park wrote:
> 
> > On Tue, 24 Feb 2026 21:28:52 -0500 Zi Yan <ziy@nvidia.com> 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@w6rz.net>
> >> Closes: https://lore.kernel.org/all/1b17c38f-30d3-4bb4-a7e1-e74b19ada885@w6rz.net/
> >> Suggested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> >> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >> ---
> >>> 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'?
> 
> Good catch. Let me fix it. Thanks.
> 
> >
> >> +
> >> +	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.
> 
> It was in free_contig_range()[1], which was called by cma_release() before.

Makes sense, thank you for clarifying, Zi :)

> 
> [1] https://elixir.bootlin.com/linux/v6.19/source/mm/page_alloc.c#L7238


Thanks,
SJ

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