Re: [PATCH v3 3/3] mm: hugetlb: fix GFP mask propagation to CMA

[Qi Zheng <qi.zheng@linux.dev>]

On 11/8/26 1:02 AM, Oscar Salvador (SUSE) wrote:
> On Sat, Nov 07, 2026 at 06:30:47PM +0800, Qi Zheng wrote:
>> From: Qi Zheng <zhengqi.arch@bytedance.com>
>>
>> cma_alloc_frozen_compound() uses GFP_KERNEL, allowing direct reclaim
>> during the first MPOL_PREFERRED_MANY allocation attempt even though
>> the caller clears __GFP_DIRECT_RECLAIM.
>>
>> Pass the caller's GFP mask through the helper so CMA respects its
>> reclaim constraints when trying the preferred nodes.
>>
>> Fixes: 9bda131c6093 ("mm: cma: add cma_alloc_frozen{_compound}()")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Acked-by: Muchun Song <muchun.song@linux.dev>
>> Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
> 
> Reviewed-by: Oscar Salvador (SUSE) <osalvador@kernel.org>
> 
> A comment below:
> 
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -933,9 +933,10 @@ struct page *cma_alloc_frozen(struct cma *cma, unsigned long count,
>>   	return __cma_alloc_frozen(cma, count, align, gfp);
>>   }
>>   
>> -struct page *cma_alloc_frozen_compound(struct cma *cma, unsigned int order)
>> +struct page *cma_alloc_frozen_compound(struct cma *cma, unsigned int order, gfp_t gfp)
>>   {
>> -	gfp_t gfp = GFP_KERNEL | __GFP_COMP | __GFP_NOWARN;
>> +	if (WARN_ON_ONCE(!(gfp & __GFP_COMP)))
>> +		return NULL;
> 
> I think this needs a comment explaining why we are checking for __GFP_COMP,
> because it might not be obvious to the casual reader.
> Something like:
> 
>   "The only caller of cma_alloc_frozen_compound() is hugetlb, and hugetlb
>    constructs its gfp mask via htlb_alloc_mask(), which already sets
>    __GFP_COMP. Check here for usecases bypassing htlb_alloc_mask()."

OK, will add this comment.

Thanks,
Qi

> 
> 
> Maybe in the future we do have some other users of this function which are outside
> of the hugetlb realm, but if we are propagating the gfp mask down the chain to this
> point, we do have to check that those users are also setting __GFP_COMP.
>    
>   
>