Re: [PATCH v4 5/6] mm: cma: add cma_alloc_frozen{_compound}()

[Zi Yan <ziy@nvidia.com>]

On 17 Dec 2025, at 3:02, Kefeng Wang wrote:

> On 2025/12/17 2:40, Zi Yan wrote:
>> On 16 Dec 2025, at 6:48, Kefeng Wang wrote:
>>
>>> Introduce cma_alloc_frozen{_compound}() helper to alloc pages without
>>> incrementing their refcount, then convert hugetlb cma to use the
>>> cma_alloc_frozen_compound() and cma_release_frozen() and remove the
>>> unused cma_{alloc,free}_folio(), also move the cma_validate_zones()
>>> into mm/internal.h since no outside user.
>>>
>>> The set_pages_refcounted() is only called to set non-compound pages
>>> after above changes, so remove the processing about PageHead.
>>>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> ---
>>>   include/linux/cma.h | 26 ++++++------------------
>>>   mm/cma.c            | 48 +++++++++++++++++++++++++--------------------
>>>   mm/hugetlb_cma.c    | 24 +++++++++++++----------
>>>   mm/internal.h       | 10 +++++-----
>>>   4 files changed, 52 insertions(+), 56 deletions(-)
>>>
>
> ...
>
>>>   static bool __cma_release(struct cma *cma, const struct page *pages,
>>> -			  unsigned long count, bool compound)
>>> +			  unsigned long count, bool frozen)
>>>   {
>>>   	unsigned long pfn, end;
>>>   	int r;
>>> @@ -974,8 +982,8 @@ static bool __cma_release(struct cma *cma, const struct page *pages,
>>>   		return false;
>>>   	}
>>>
>>> -	if (compound)
>>> -		__free_pages((struct page *)pages, compound_order(pages));
>>> +	if (frozen)
>>> +		free_contig_frozen_range(pfn, count);
>>>   	else
>>>   		free_contig_range(pfn, count);
>>
>> Can we get rid of free_contig_range() branch by making cma_release() put
>> each page’s refcount? Then, __cma_relase() becomes cma_release_frozen()
>> and the release pattern matches allocation pattern:
>> 1. cma_alloc() calls cma_alloc_frozen() and manipulates page refcount.
>> 2. cma_release() manipulates page refcount and calls cma_release_frozen().
>>
>
> Have considered similar things before, but we need manipulates page
> refcount only find the correct cma memrange from cma/pages, it seems
> that no big improvement, any more comments?
>
> 1) for cma_release:
>    a. cma find memrange
>    b. manipulates page refcount when cmr found
>    c. free page and release cma resource
> 2) for cma_release_frozen
>    a. cma find memrange
>    b. free page and release cma resource whne cmr found

Right, I think it makes code simpler.

Basically add a helper function:
struct cma_memrange* find_cma_memrange(struct cma *cma,
		const struct page *pages, unsigned long count);

Then

__cma_release_frozen()
{
	free_contig_frozen_range(pfn, count);
	cma_clear_bitmap(cma, cmr, pfn, count);
	cma_sysfs_account_release_pages(cma, count);
	trace_cma_release(cma->name, pfn, pages, count);
}


cma_release()
{
	cmr = find_cma_memrange();

	if (!cmr)
		return false;
	
	for (; count--; pages++)
		VM_WARN_ON(!put_page_testzero(pages);

	__cma_release_frozen();
}

cma_release_frozen()
{
	cmr = find_cma_memrange();

	if (!cmr)
		return false;

	__cma_release_frozen();

}

Let me know your thoughts.


Best Regards,
Yan, Zi