Re: [PATCH v4 3/3] mm: hugetlb: fix GFP mask propagation to CMA

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

On 11/12/26 7:27 PM, Kefeng Wang wrote:
> 
> 
> On 11/11/2026 2:46 PM, 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.
> 
> MPOL_PREFERRED_MANY is only used for the partner allocator. Without your

It's doesn't seem so.

Gigantic surplus allocations already use MPOL_PREFERRED_MANY, following
eb02f14c4a2b ("mm/hugetlb: allow overcommitting gigantic hugepages"),
which predates 9bda131c6093.

alloc_buddy_hugetlb_folio
--> MPOL_PREFERRED_MANY
     --> gfp_t gfp = gfp_mask & ~(__GFP_DIRECT_RECLAIM | __GFP_NOFAIL);
         folio = alloc_surplus_hugetlb_folio(h, gfp, mpoli->nid, nodemask);
         --> alloc_fresh_hugetlb_folio
             --> only_alloc_fresh_hugetlb_folio
                 --> alloc_gigantic_frozen_folio
                     --> hugetlb_cma_alloc_frozen_folio
                         --> cma_alloc_frozen_compound

Before 9bda131c6093, cma_alloc_folio() forwarded the caller's GFP mask.
The conversion to cma_alloc_frozen_compound() replaced it with
GFP_KERNEL, re-enabling direct reclaim during the preferred-node
attempt.

Right?

Thanks,
Qi

> patches "mm: hugetlb: add CMA support for PMD-sized pages,"
> cma_alloc_frozen_compound() is only used for huge page allocations. In
> this case, I think we don't need the gfp flag, which is why I hardcoded
> it, so there's no need for the Fixes tag; this patch should be part of
> your new feature. Maybe I missed something?
> 
>>
>> 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>
>> ---
>>   include/linux/cma.h |  2 +-
>>   mm/cma.c            | 10 ++++++++--
>>   mm/hugetlb_cma.c    |  4 ++--
>>   3 files changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/cma.h b/include/linux/cma.h
>> index 8555d38a97b1a..4539ea5f2c4e8 100644
>> --- a/include/linux/cma.h
>> +++ b/include/linux/cma.h
>> @@ -53,7 +53,7 @@ extern bool cma_release(struct cma *cma, const 
>> struct page *pages, unsigned long
>>   struct page *cma_alloc_frozen(struct cma *cma, unsigned long count,
>>           unsigned int align, bool no_warn);
>> -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);
>>   bool cma_release_frozen(struct cma *cma, const struct page *pages,
>>           unsigned long count);
>> diff --git a/mm/cma.c b/mm/cma.c
>> index 9ec1d1fd5dff5..a7c47c0e7d070 100644
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -933,9 +933,15 @@ 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;
>> +    /*
>> +     * 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().
>> +     */
>> +    if (WARN_ON_ONCE(!(gfp & __GFP_COMP)))
>> +        return NULL;
>>       return __cma_alloc_frozen(cma, 1 << order, order, gfp);
>>   }
>> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
>> index cd6a64986278a..3e5a30571524a 100644
>> --- a/mm/hugetlb_cma.c
>> +++ b/mm/hugetlb_cma.c
>> @@ -69,14 +69,14 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int 
>> order, gfp_t gfp_mask,
>>       }
>>       if (hugetlb_cma[nid] && node_isset(nid, *nmask))
>> -        page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);
>> +        page = cma_alloc_frozen_compound(hugetlb_cma[nid], order, 
>> gfp_mask);
>>       if (!page && !(gfp_mask & __GFP_THISNODE)) {
>>           for_each_node_mask(node, *nmask) {
>>               if (node == nid || !hugetlb_cma[node])
>>                   continue;
>> -            page = cma_alloc_frozen_compound(hugetlb_cma[node], order);
>> +            page = cma_alloc_frozen_compound(hugetlb_cma[node], 
>> order, gfp_mask);
>>               if (page)
>>                   break;
>>           }
>