[PATCH v2 1/2] selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results

[David Hildenbrand <david@redhat.com>]

When checking for actual tail or head pages of a folio, we must make
sure that the KPF_COMPOUND_HEAD/KPF_COMPOUND_TAIL flag is paired with
KPF_THP.

For example, if we have another large folio after our large folio in
physical memory, our "pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL)" would
trigger even though it's actually a head page of the next folio.

If is_backed_by_folio() returns a wrong result, split_pte_mapped_thp()
can fail with "Some THPs are missing during mremap".

Fix it by checking for head/tail pages of folios properly. Add
folio_tail_flags/folio_head_flags to improve readability and use these
masks also when just testing for any compound page.

Fixes: 169b456b0162 ("selftests/mm: reimplement is_backed_by_thp() with more precise check")
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 tools/testing/selftests/mm/split_huge_page_test.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index 10ae65ea032f6..72d6d8bb329ed 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -44,6 +44,8 @@ int kpageflags_fd;
 static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 		int kpageflags_fd)
 {
+	const uint64_t folio_head_flags = KPF_THP | KPF_COMPOUND_HEAD;
+	const uint64_t folio_tail_flags = KPF_THP | KPF_COMPOUND_TAIL;
 	const unsigned long nr_pages = 1UL << order;
 	unsigned long pfn_head;
 	uint64_t pfn_flags;
@@ -61,7 +63,7 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 
 	/* check for order-0 pages */
 	if (!order) {
-		if (pfn_flags & (KPF_THP | KPF_COMPOUND_HEAD | KPF_COMPOUND_TAIL))
+		if (pfn_flags & (folio_head_flags | folio_tail_flags))
 			return false;
 		return true;
 	}
@@ -76,14 +78,14 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 		goto fail;
 
 	/* head PFN has no compound_head flag set */
-	if (!(pfn_flags & (KPF_THP | KPF_COMPOUND_HEAD)))
+	if ((pfn_flags & folio_head_flags) != folio_head_flags)
 		return false;
 
 	/* check all tail PFN flags */
 	for (i = 1; i < nr_pages; i++) {
 		if (pageflags_get(pfn_head + i, kpageflags_fd, &pfn_flags))
 			goto fail;
-		if (!(pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL)))
+		if ((pfn_flags & folio_tail_flags) != folio_tail_flags)
 			return false;
 	}
 
@@ -94,11 +96,8 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 	if (pageflags_get(pfn_head + nr_pages, kpageflags_fd, &pfn_flags))
 		return true;
 
-	/* this folio is bigger than the given order */
-	if (pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL))
-		return false;
-
-	return true;
+	/* If we find another tail page, then the folio is larger. */
+	return (pfn_flags & folio_tail_flags) != folio_tail_flags;
 fail:
 	ksft_exit_fail_msg("Failed to get folio info\n");
 	return false;
-- 
2.50.1