Re: [PATCH v3 3/4] selftests/mm: reimplement is_backed_by_thp() with more precise check

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Aug 12, 2025 at 11:55:11AM -0400, Zi Yan wrote:
>and rename it to is_backed_by_folio().
>
>is_backed_by_folio() checks if the given vaddr is backed a folio with
>a given order. It does so by:
>1. getting the pfn of the vaddr;
>2. checking kpageflags of the pfn;
>
>if order is greater than 0:
>3. checking kpageflags of the head pfn;
>4. checking kpageflags of all tail pfns.
>
>pmd_order is added to split_huge_page_test.c and replaces max_order.
>
>Signed-off-by: Zi Yan <ziy@nvidia.com>
>---
> .../selftests/mm/split_huge_page_test.c       | 67 +++++++++++++------
> tools/testing/selftests/mm/vm_util.c          |  2 +-
> tools/testing/selftests/mm/vm_util.h          |  1 +
> 3 files changed, 48 insertions(+), 22 deletions(-)
>
>diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
>index 63ac82f0b9e0..3aaf783f339f 100644
>--- a/tools/testing/selftests/mm/split_huge_page_test.c
>+++ b/tools/testing/selftests/mm/split_huge_page_test.c
>@@ -25,6 +25,7 @@
> uint64_t pagesize;
> unsigned int pageshift;
> uint64_t pmd_pagesize;
>+unsigned int pmd_order;
> 
> #define SPLIT_DEBUGFS "/sys/kernel/debug/split_huge_pages"
> #define SMAP_PATH "/proc/self/smaps"
>@@ -36,23 +37,48 @@ uint64_t pmd_pagesize;
> 
> #define GET_ORDER(nr_pages)    (31 - __builtin_clz(nr_pages))
> 
>-int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file)
>+int is_backed_by_folio(char *vaddr, int order, int pagemap_fd, int kpageflags_fd)
> {
>-	uint64_t paddr;
>-	uint64_t page_flags;
>+	unsigned long pfn_head;
>+	uint64_t pfn_flags;
>+	unsigned long pfn;
>+	unsigned long i;
> 
>-	if (pagemap_file) {
>-		pread(pagemap_file, &paddr, sizeof(paddr),
>-			((long)vaddr >> pageshift) * sizeof(paddr));
>+	if (!pagemap_fd || !kpageflags_fd)
>+		return 0;

The same in patch 2.

> 
>-		if (kpageflags_file) {
>-			pread(kpageflags_file, &page_flags, sizeof(page_flags),
>-				PAGEMAP_PFN(paddr) * sizeof(page_flags));
>+	pfn = pagemap_get_pfn(pagemap_fd, vaddr);
> 
>-			return !!(page_flags & KPF_THP);
>-		}
>+	if (pfn == -1UL)
>+		return 0;
>+
>+	if (get_pfn_flags(pfn, kpageflags_fd, &pfn_flags))
>+		return 0;
>+
>+	if (!order) {
>+		if (pfn_flags & (KPF_THP | KPF_COMPOUND_HEAD | KPF_COMPOUND_TAIL))
>+			return 0;
>+		return 1;
> 	}
>-	return 0;
>+
>+	if (!(pfn_flags & KPF_THP))
>+		return 0;
>+
>+	pfn_head = pfn & ~((1 << order) - 1);
>+
>+	if (get_pfn_flags(pfn_head, kpageflags_fd, &pfn_flags))
>+		return 0;
>+
>+	if (!(pfn_flags & (KPF_THP | KPF_COMPOUND_HEAD)))
>+		return 0;
>+
>+	for (i = 1; i < (1UL << order) - 1; i++) {

Do we miss the last tail?

>+		if (get_pfn_flags(pfn_head + i, kpageflags_fd, &pfn_flags))
>+			return 0;
>+		if (!(pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL)))
>+			return 0;
>+	}

If this folio is larger than order, would it still return 1?

>+	return 1;
> }
> 
> static void write_file(const char *path, const char *buf, size_t buflen)
>@@ -233,7 +259,7 @@ void split_pte_mapped_thp(void)
> 	thp_size = 0;
> 	for (i = 0; i < pagesize * 4; i++)
> 		if (i % pagesize == 0 &&
>-		    is_backed_by_thp(&pte_mapped[i], pagemap_fd, kpageflags_fd))
>+		    is_backed_by_folio(&pte_mapped[i], pmd_order, pagemap_fd, kpageflags_fd))
> 			thp_size++;
> 
> 	if (thp_size != 4)
>@@ -250,7 +276,7 @@ void split_pte_mapped_thp(void)
> 			ksft_exit_fail_msg("%ld byte corrupted\n", i);
> 
> 		if (i % pagesize == 0 &&
>-		    is_backed_by_thp(&pte_mapped[i], pagemap_fd, kpageflags_fd))
>+		    !is_backed_by_folio(&pte_mapped[i], 0, pagemap_fd, kpageflags_fd))
> 			thp_size++;
> 	}
> 
>@@ -522,7 +548,6 @@ int main(int argc, char **argv)
> 	const char *fs_loc;
> 	bool created_tmp;
> 	int offset;
>-	unsigned int max_order;
> 	unsigned int nr_pages;
> 	unsigned int tests;
> 
>@@ -543,28 +568,28 @@ int main(int argc, char **argv)
> 		ksft_exit_fail_msg("Reading PMD pagesize failed\n");
> 
> 	nr_pages = pmd_pagesize / pagesize;
>-	max_order = GET_ORDER(nr_pages);
>-	tests = 2 + (max_order - 1) + (2 * max_order) + (max_order - 1) * 4 + 2;
>+	pmd_order = GET_ORDER(nr_pages);
>+	tests = 2 + (pmd_order - 1) + (2 * pmd_order) + (pmd_order - 1) * 4 + 2;
> 	ksft_set_plan(tests);
> 
> 	fd_size = 2 * pmd_pagesize;
> 
> 	split_pmd_zero_pages();
> 
>-	for (i = 0; i < max_order; i++)
>+	for (i = 0; i < pmd_order; i++)
> 		if (i != 1)
> 			split_pmd_thp_to_order(i);
> 
> 	split_pte_mapped_thp();
>-	for (i = 0; i < max_order; i++)
>+	for (i = 0; i < pmd_order; i++)
> 		split_file_backed_thp(i);
> 
> 	created_tmp = prepare_thp_fs(optional_xfs_path, fs_loc_template,
> 			&fs_loc);
>-	for (i = max_order - 1; i >= 0; i--)
>+	for (i = pmd_order - 1; i >= 0; i--)
> 		split_thp_in_pagecache_to_order_at(fd_size, fs_loc, i, -1);
> 
>-	for (i = 0; i < max_order; i++)
>+	for (i = 0; i < pmd_order; i++)
> 		for (offset = 0;
> 		     offset < nr_pages;
> 		     offset += MAX(nr_pages / 4, 1 << i))
>diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
>index 4d952d1bc96d..193ba1a1a3cc 100644
>--- a/tools/testing/selftests/mm/vm_util.c
>+++ b/tools/testing/selftests/mm/vm_util.c
>@@ -338,7 +338,7 @@ int detect_hugetlb_page_sizes(size_t sizes[], int max)
> 	return count;
> }
> 
>-static int get_pfn_flags(unsigned long pfn, int kpageflags_fd, uint64_t *flags)
>+int get_pfn_flags(unsigned long pfn, int kpageflags_fd, uint64_t *flags)
> {
> 	size_t count;
> 
>diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
>index 02e3f1e7065b..148b792cff0f 100644
>--- a/tools/testing/selftests/mm/vm_util.h
>+++ b/tools/testing/selftests/mm/vm_util.h
>@@ -92,6 +92,7 @@ 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 get_pfn_flags(unsigned long pfn, int kpageflags_fd, uint64_t *flags);
> 
> int uffd_register(int uffd, void *addr, uint64_t len,
> 		  bool miss, bool wp, bool minor);
>-- 
>2.47.2

-- 
Wei Yang
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