Re: [PATCH v2 2/3] selftests/mm: add check_folio_orders() helper.

On 9 Aug 2025, at 16:18, Wei Yang wrote:

On Fri, Aug 08, 2025 at 03:01:43PM -0400, Zi Yan wrote:

...

The helper gathers an folio order statistics of folios within a virtual address range and checks it against a given order list. It aims to provide a more precise folio order check instead of just checking the existence of PMD folios.

Signed-off-by: Zi Yan ziy@nvidia.com

.../selftests/mm/split_huge_page_test.c | 4 +- tools/testing/selftests/mm/vm_util.c | 133 ++++++++++++++++++ tools/testing/selftests/mm/vm_util.h | 7 + 3 files changed, 141 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c index cb364c5670c6..5ab488fab1cd 100644 --- a/tools/testing/selftests/mm/split_huge_page_test.c +++ b/tools/testing/selftests/mm/split_huge_page_test.c @@ -34,8 +34,6 @@ uint64_t pmd_pagesize; #define PID_FMT_OFFSET "%d,0x%lx,0x%lx,%d,%d" #define PATH_FMT "%s,0x%lx,0x%lx,%d"

-#define PFN_MASK ((1UL<<55)-1) -#define KPF_THP (1UL<<22) #define GET_ORDER(nr_pages) (31 - __builtin_clz(nr_pages))

int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file) @@ -49,7 +47,7 @@ int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file)

if (kpageflags_file) {
	pread(kpageflags_file, &page_flags, sizeof(page_flags),

• 		(paddr & PFN_MASK) * sizeof(page_flags));
  

• 		PAGEMAP_PFN(paddr) * sizeof(page_flags));
  
  

is_backed_by_thp() shares similar logic as get_page_flags(), I am thinking we can leverage get_page_flags() here.

I was lazy for this one. I will use check_folio_orders() in the next version.

...

	return !!(page_flags & KPF_THP);
}

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c index 6a239aa413e2..41d50b74b2f6 100644 --- a/tools/testing/selftests/mm/vm_util.c +++ b/tools/testing/selftests/mm/vm_util.c @@ -338,6 +338,139 @@ int detect_hugetlb_page_sizes(size_t sizes[], int max) return count; }

+static int get_page_flags(char *vaddr, int pagemap_file, int kpageflags_file,

• 	  uint64_t *flags)
  

+{

Nit.

In vm_util.c, we usually name the file descriptor as xxx_fd.

OK. I can rename them.

...

• unsigned long pfn;
• size_t count;
• pfn = pagemap_get_pfn(pagemap_file, vaddr);
• /*

• * Treat non-present page as a page without any flag, so that
  

• * gather_folio_orders() just record the current folio order.
  

• */
  

• if (pfn == -1UL) {

• *flags = 0;
  

• return 0;
  

• }
• count = pread(kpageflags_file, flags, sizeof(*flags),

•       pfn * sizeof(*flags));
  

• if (count != sizeof(*flags))

• return -1;
  

• return 0;

+}

Maybe a simple document here would be helpful.

Will do.

...

+static int gather_folio_orders(char *vaddr_start, size_t len,

• 	       int pagemap_file, int kpageflags_file,
  

• 	       int orders[], int nr_orders)
  

+{

• uint64_t page_flags = 0;
• int cur_order = -1;
• char *vaddr;
• if (!pagemap_file || !kpageflags_file)

• return -1;
  

• if (nr_orders <= 0)

• return -1;
  

• for (vaddr = vaddr_start; vaddr < vaddr_start + len; ) {

• char *next_folio_vaddr;
  

• int status;
  

• if (get_page_flags(vaddr, pagemap_file, kpageflags_file, &page_flags))
  

• 	return -1;
  

• /* all order-0 pages with possible false postive (non folio) */
  

• if (!(page_flags & (KPF_COMPOUND_HEAD | KPF_COMPOUND_TAIL))) {
  

• 	orders[0]++;
  

• 	vaddr += psize();
  

• 	continue;
  

• }
  

• /* skip non thp compound pages */
  

• if (!(page_flags & KPF_THP)) {
  

• 	vaddr += psize();
  

• 	continue;
  

• }
  

• /* vpn points to part of a THP at this point */
  

• if (page_flags & KPF_COMPOUND_HEAD)
  

• 	cur_order = 1;
  

• else {
  

• 	/* not a head nor a tail in a THP? */
  

• 	if (!(page_flags & KPF_COMPOUND_TAIL))
  

• 		return -1;
  

• 	continue;
  

• }
  

• next_folio_vaddr = vaddr + (1UL << (cur_order + pshift()));
  

• if (next_folio_vaddr >= vaddr_start + len)
  

• 	break;
  

Would we skip order 1 folio at the last position?

For example, vaddr_start is 0x2000, len is 0x2000 and the folio at vaddr_start is an order 1 folio, whose size is exactly 0x2000.

Then we will get next_folio_vaddr == vaddr_start + len.

Could that happen?

No. After the loop, there is code checking cur_order and updating orders[].

...

• while (!(status = get_page_flags(next_folio_vaddr, pagemap_file,
  

• 				 kpageflags_file,
  

• 				 &page_flags))) {
  

• 	/* next compound head page or order-0 page */
  

• 	if ((page_flags & KPF_COMPOUND_HEAD) ||
  

• 	    !(page_flags & (KPF_COMPOUND_HEAD |
  

• 	      KPF_COMPOUND_TAIL))) {
  

Maybe we can put them into one line.

Sure.

...

• 		if (cur_order < nr_orders) {
  

• 			orders[cur_order]++;
  

• 			cur_order = -1;
  

• 			vaddr = next_folio_vaddr;
  

• 		}
  

• 		break;
  

• 	}
  

• 	/* not a head nor a tail in a THP? */
  

• 	if (!(page_flags & KPF_COMPOUND_TAIL))
  

• 		return -1;
  

• 	cur_order++;
  

• 	next_folio_vaddr = vaddr + (1UL << (cur_order + pshift()));
  

• }
  

The while loop share similar logic as the outer for loop. Is it possible reduce some duplication?

Outer loop is to filter order-0 and non head pages and while loop is to find current THP/mTHP orders. It would be messy to combine them. But feel free to provide ideas if you see a way.

...

• if (status)
  

• 	return status;
  

• }
• if (cur_order > 0 && cur_order < nr_orders)

• orders[cur_order]++;
  

• return 0;

+}

+int check_folio_orders(char *vaddr_start, size_t len, int pagemap_file,

• 	int kpageflags_file, int orders[], int nr_orders)
  

+{

• int *vaddr_orders;
• int status;
• int i;
• vaddr_orders = (int *)malloc(sizeof(int) * nr_orders);

I took a look into thp_setting.h, where defines an array with NR_ORDERS element which is 20. Maybe we can leverage it here, since we don't expect the order to be larger.

20 is too large for current use. We can revisit this when the function gets more users.

...

• if (!vaddr_orders)

• ksft_exit_fail_msg("Cannot allocate memory for vaddr_orders");
  

• memset(vaddr_orders, 0, sizeof(int) * nr_orders);
• status = gather_folio_orders(vaddr_start, len, pagemap_file,

• 		     kpageflags_file, vaddr_orders, nr_orders);
  

• if (status)

• return status;
  

• status = 0;
• for (i = 0; i < nr_orders; i++)

• if (vaddr_orders[i] != orders[i]) {
  

• 	ksft_print_msg("order %d: expected: %d got %d\n", i,
  

• 		       orders[i], vaddr_orders[i]);
  

• 	status = -1;
  

• }
  

• return status;

+}

/* If `ioctls' non-NULL, the allowed ioctls will be returned into the var */ int uffd_register_with_ioctls(int uffd, void *addr, uint64_t len, bool miss, bool wp, bool minor, uint64_t *ioctls) diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h index 1843ad48d32b..02e3f1e7065b 100644 --- a/tools/testing/selftests/mm/vm_util.h +++ b/tools/testing/selftests/mm/vm_util.h @@ -18,6 +18,11 @@ #define PM_SWAP BIT_ULL(62) #define PM_PRESENT BIT_ULL(63)

+#define KPF_COMPOUND_HEAD BIT_ULL(15) +#define KPF_COMPOUND_TAIL BIT_ULL(16) +#define KPF_THP BIT_ULL(22)

/*

• Ignore the checkpatch warning, we must read from x but don't want to do
• anything with it in order to trigger a read page fault. We therefore must use

@@ -85,6 +90,8 @@ bool check_huge_shmem(void *addr, int nr_hpages, uint64_t hpage_size); int64_t allocate_transhuge(void *ptr, int pagemap_fd); unsigned long default_huge_page_size(void); int detect_hugetlb_page_sizes(size_t sizes[], int max); +int check_folio_orders(char *vaddr_start, size_t len, int pagemap_file,

• 	int kpageflags_file, int orders[], int nr_orders);
  
  

int uffd_register(int uffd, void *addr, uint64_t len, bool miss, bool wp, bool minor); -- 2.47.2

-- Wei Yang Help you, Help me

Best Regards, Yan, Zi