[PATCH v4 08/19] selftests/mm: add folio-order check for address ranges

From: "Kiryl Shutsemau (Meta)" <[email protected]>

An mTHP collapse test needs to know that a range is backed by folios of the
target order, and that they sit where a collapse would put them.  Nothing
answers that today: is_backed_by_folio() classifies the folio behind a
single page, and check_huge_anon() reads smaps AnonHugePages, which only
accounts PMD mappings.

Add is_range_backed_by_folio_orders().  For every order-aligned window of
the range it requires a present head PFN at its natural alignment and a
contiguous PFN run across the window.

A window backed by two smaller folios fails the contiguity check, and a
folio mapped off the window's alignment fails the head check.  The mTHP
cases need both to tell a collapsed window from the one beside it.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <[email protected]>
Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]>
---
 tools/testing/selftests/mm/vm_util.c | 42 ++++++++++++++++++++++++++++
 tools/testing/selftests/mm/vm_util.h |  2 ++
 2 files changed, 44 insertions(+)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index 5db1a7774f49..c9bd6c92fa41 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -556,6 +556,48 @@ bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 	return false;
 }
 
+/*
+ * Check whether every order-@order window of [start, len) maps exactly one
+ * folio of that order, head to tail.  The address range must be naturally
+ * aligned, each window's PFN run must be contiguous, and a window's first
+ * PFN must be the folio head.
+ *
+ * This is the check "did this range collapse into order-@order folios": a
+ * window assembled from parts of several folios, or mapping a folio shifted
+ * from its natural position, fails.
+ */
+bool is_range_backed_by_folio_orders(char *start, size_t len, int order,
+				     int pagemap_fd, int kpageflags_fd)
+{
+	const unsigned long nr_pages = 1UL << order;
+	const size_t window = nr_pages * psize();
+	char *vaddr;
+
+	if ((uintptr_t)start % window || len % window)
+		return false;
+
+	for (vaddr = start; vaddr < start + len; vaddr += window) {
+		unsigned long pfn = pagemap_get_pfn(pagemap_fd, vaddr);
+		unsigned long i;
+
+		/* Not present, or not mapping the folio head. */
+		if (pfn == -1UL || pfn % nr_pages)
+			return false;
+
+		for (i = 1; i < nr_pages; i++) {
+			if (pagemap_get_pfn(pagemap_fd, vaddr + i * psize()) !=
+			    pfn + i)
+				return false;
+		}
+
+		if (!is_backed_by_folio(vaddr, order, pagemap_fd,
+					kpageflags_fd))
+			return false;
+	}
+
+	return true;
+}
+
 /* 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 56a28ce7d029..39dfb18dc10c 100644
--- a/tools/testing/selftests/mm/vm_util.h
+++ b/tools/testing/selftests/mm/vm_util.h
@@ -99,6 +99,8 @@ int gather_folio_orders(char *vaddr_start, size_t len,
 		int pagemap_fd, int kpageflags_fd, int orders[], int nr_orders);
 bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 			int kpageflags_fd);
+bool is_range_backed_by_folio_orders(char *start, size_t len, int order,
+				     int pagemap_fd, int kpageflags_fd);
 
 int uffd_register(int uffd, void *addr, uint64_t len,
 		  bool miss, bool wp, bool minor);
-- 
2.54.0