Re: [PATCH 2/4] mm: hugetlb: skip CMA allocations under PF_MEMALLOC_PIN

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On 11/3/26 11:56 AM, Muchun Song wrote:
> 
> 
>> On Nov 3, 2026, at 16:01, Qi Zheng <qi.zheng@linux.dev> wrote:
>>
>> From: Qi Zheng <zhengqi.arch@bytedance.com>
>>
>> With gigantic overcommit enabled, a fault during long-term pinning
>> can allocate a fresh HugeTLB folio from CMA despite PF_MEMALLOC_PIN.
>>
>> The folio must then be migrated before it can be pinned. However,
>> fresh gigantic migration targets are not supported, so pinning fails
>> when the HugeTLB pool has no suitable target, even if the original
>> fault could have allocated pinnable memory outside CMA.
>>
>> Honor PF_MEMALLOC_PIN in the HugeTLB CMA allocator so the fault can
>> try the non-CMA fallback, or fail when allocation is restricted to CMA.
> 
> Wait... I think my Acked-by got ahead of me here. :)
> 
> There seems to be a rather odd distinction that GUP is missing.
> 
> The usual reason long-term GUP must migrate a page out of a
> MIGRATE_CMA pageblock is that the page was only borrowed from CMA by the
> normal page allocator.  The corresponding CMA bitmap range is still
> available, so a future CMA allocation relies on being able to migrate
> that page away.  A long-term pin would prevent that.
> 
> However, a HugeTLB folio returned by
> hugetlb_cma_alloc_frozen_folio() is different.  It is the actual CMA
> allocation, not merely a movable page that happens to reside in a
> MIGRATE_CMA pageblock.  Its range has already been marked allocated in
> the hugetlb CMA bitmap.
> 
> Therefore, CMA cannot allocate that range again until the HugeTLB folio
> is freed anyway.  Long-term pinning the folio does not seem to make the
> CMA area any less available than keeping the HugeTLB folio mapped for
> the same amount of time.
> 
> folio_is_longterm_pinnable() currently only sees MIGRATE_CMA and does
> not distinguish these two cases.  In that light, rejecting
> PF_MEMALLOC_PIN in hugetlb_cma_alloc_frozen_folio() looks like working
> around an overly broad GUP classification.  It also means that
> hugetlb_cma_only users will fail the fault, while other users may consume
> a gigantic contiguous range outside the dedicated hugetlb CMA area even
> when that area has free space.
> 
> Would it make more sense to fix the long-term GUP check instead, so that
> an explicitly CMA-allocated HugeTLB folio is considered pinnable, while
> ordinary pages merely borrowed from MIGRATE_CMA pageblocks remain unpinnable?
> 

makes sense to me. with hugetlb_cma_only=1 a 1G FOLL_LONGTERM pin can't
succeed at all today, even on preallocated pages.


-- 
cheers,
jose a. p-a