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

[Muchun Song]

> On Nov 4, 2026, at 11:27, Qi Zheng <qi.zheng@xxxxxxxxx> wrote:
> 
> Hi Muchun,

Hi,

> 
> On 11/3/26 6:56 PM, Muchun Song wrote:
>>> On Nov 3, 2026, at 16:01, Qi Zheng <qi.zheng@xxxxxxxxx> wrote:
>>> 
>>> From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
>>> 
>>> 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?
> 
> Make sense!
> 
> I'll replace this patch with an exception for HPG_cma folios in the
> MIGRATE_CMA check. This also lets the HugeTLB allocator select preallocated
> CMA folios during long-term pinning. MIGRATE_ISOLATE and ZONE_MOVABLE
> remain restricted.
> 
> Fresh CMA allocations under PF_MEMALLOC_PIN also need to skip ranges
> in ZONE_MOVABLE. Returning -EBUSY here lets CMA try its remaining ranges.
> 
> The incremental diff against this patch is below.
> 
> diff --git a/include/linux/cma.h b/include/linux/cma.h
> index 8555d38a97b1..7fd1b6e65b58 100644
> --- a/include/linux/cma.h
> +++ b/include/linux/cma.h
> @@ -21,11 +21,13 @@
> #define CMA_MIN_ALIGNMENT_BYTES (PAGE_SIZE * CMA_MIN_ALIGNMENT_PAGES)
> 
> struct cma;
> +struct folio;
> 
> extern unsigned long totalcma_pages;
> extern phys_addr_t cma_get_base(const struct cma *cma);
> extern unsigned long cma_get_size(const struct cma *cma);
> extern const char *cma_get_name(const struct cma *cma);
> +bool cma_folio_is_pinnable(struct folio *folio);
> 
> extern int __init cma_declare_contiguous_nid(phys_addr_t base,
> 			phys_addr_t size, phys_addr_t limit,
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index cb993a1d0728..d15810be7d96 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3,6 +3,7 @@
> #define _LINUX_MM_H
> 
> #include <linux/args.h>
> +#include <linux/cma.h>
> #include <linux/errno.h>
> #include <linux/mmdebug.h>
> #include <linux/gfp.h>
> @@ -2975,14 +2976,16 @@ static inline bool is_zero_folio(const struct folio *folio)
> 	return is_zero_page(&folio->page);
> }
> 
> -/* MIGRATE_CMA and ZONE_MOVABLE do not allow pin folios */
> +/* Borrowed CMA pages and ZONE_MOVABLE folios cannot be pinned long term. */
> #ifdef CONFIG_MIGRATION
> static inline bool folio_is_longterm_pinnable(struct folio *folio)
> {
> #ifdef CONFIG_CMA
> 	int mt = folio_migratetype(folio);
> 
> -	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> +	if (mt == MIGRATE_ISOLATE)
> +		return false;
> +	if (mt == MIGRATE_CMA && !cma_folio_is_pinnable(folio))
> 		return false;

Actually, we do not need cma_folio_is_pinnable(), does the following work for you?

--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2982,6 +2982,9 @@ static inline bool folio_is_longterm_pinnable(struct folio *folio)
 #ifdef CONFIG_CMA
        int mt = folio_migratetype(folio);

+       if (folio_test_hugetlb(folio) && folio_test_hugetlb_cma(folio))
+               return true;
+
        if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
                return false;
 #endif

> #endif
> /* The zero page can be "pinned" but gets special handling. */
> diff --git a/mm/cma.c b/mm/cma.c
> index a10ea37a261d..c2814f54f86a 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -27,6 +27,7 @@
> #include <linux/log2.h>
> #include <linux/cma.h>
> #include <linux/highmem.h>
> +#include <linux/hugetlb.h>
> #include <linux/io.h>
> #include <linux/kmemleak.h>
> #include <trace/events/cma.h>
> @@ -55,6 +56,12 @@ const char *cma_get_name(const struct cma *cma)
> }
> EXPORT_SYMBOL_GPL(cma_get_name);
> 
> +bool cma_folio_is_pinnable(struct folio *folio)
> +{
> +	/* Explicit HugeTLB CMA allocations own their bitmap ranges. */
> +	return folio_test_hugetlb(folio) && folio_test_hugetlb_cma(folio);
> +}
> +
> static unsigned long cma_bitmap_aligned_mask(const struct cma *cma,
> 					     unsigned int align_order)
> {
> @@ -792,6 +799,11 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
> 	int ret = -EBUSY;
> 	struct page *page = NULL;
> 
> + 	/* Each range lies within one zone; try other ranges if needed. */
> +	if ((current->flags & PF_MEMALLOC_PIN) &&
> + 	    is_zone_movable_page(pfn_to_page(cmr->base_pfn)))
> +		return -EBUSY;

I don't know why we need this.

Thanks,
Muchun

> +
> 	mask = cma_bitmap_aligned_mask(cma, align);
> 	offset = cma_bitmap_aligned_offset(cma, cmr, align);
> 	bitmap_maxno = cma_bitmap_maxno(cma, cmr);
> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
> index dde1c866406f..95fd2d190f0d 100644
> --- a/mm/hugetlb_cma.c
> +++ b/mm/hugetlb_cma.c
> @@ -62,9 +62,6 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
> 	if (!hugetlb_cma_size)
> 		return NULL;
> 
> -	if (current->flags & PF_MEMALLOC_PIN)
> -		return NULL;
> -
> retry_cpuset:
> 	if (!nodemask) {
> 		cpuset_mems_cookie = read_mems_allowed_begin();
> 
> Thanks,
> Qi
> 
>> Thanks,
>> Muchun
>>> 
>>> Fixes: eb02f14c4a2b ("mm/hugetlb: allow overcommitting gigantic hugepages")
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
>>> ---
>>> mm/hugetlb_cma.c | 3 +++
>>> 1 file changed, 3 insertions(+)
>>> 
>>> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
>>> index 95fd2d190f0d2..dde1c866406fb 100644
>>> --- a/mm/hugetlb_cma.c
>>> +++ b/mm/hugetlb_cma.c
>>> @@ -62,6 +62,9 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
>>> 	if (!hugetlb_cma_size)
>>> 		return NULL;
>>> 
>>> +	if (current->flags & PF_MEMALLOC_PIN)
>>> +		return NULL;
>>> +
>>> retry_cpuset:
>>> 	if (!nodemask) {
>>> 		cpuset_mems_cookie = read_mems_allowed_begin();
>>> -- 
>>> 2.54.0