+ selftests-mm-split_huge_page_test-fix-occasional-is_backed_by_folio-wrong-results.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results
has been added to the -mm mm-new branch.  Its filename is
     selftests-mm-split_huge_page_test-fix-occasional-is_backed_by_folio-wrong-results.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-split_huge_page_test-fix-occasional-is_backed_by_folio-wrong-results.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results
Date: Wed, 3 Sep 2025 09:02:52 +0200

Patch series "selftests/mm: split_huge_page_test: split_pte_mapped_thp
improvements", v2.

One fix for occasional failures I found while testing and a bunch of
cleanups that should make that test easier to digest.


This patch (of 2):

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.

Link: https://lkml.kernel.org/r/20250903070253.34556-1-david@redhat.com
Link: https://lkml.kernel.org/r/20250903070253.34556-2-david@redhat.com
Fixes: 169b456b0162 ("selftests/mm: reimplement is_backed_by_thp() with more precise check")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 tools/testing/selftests/mm/split_huge_page_test.c |   15 +++++-------
 1 file changed, 7 insertions(+), 8 deletions(-)

--- a/tools/testing/selftests/mm/split_huge_page_test.c~selftests-mm-split_huge_page_test-fix-occasional-is_backed_by_folio-wrong-results
+++ a/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 *vad
 
 	/* 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 *vad
 		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 *vad
 	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;
_

Patches currently in -mm which might be from david@redhat.com are

mm-migrate-remove-migratepage_unmap.patch
mm-migrate-remove-migratepage_unmap-fix.patch
treewide-remove-migratepage_success.patch
mm-huge_memory-move-more-common-code-into-insert_pmd.patch
mm-huge_memory-move-more-common-code-into-insert_pud.patch
mm-huge_memory-support-huge-zero-folio-in-vmf_insert_folio_pmd.patch
fs-dax-use-vmf_insert_folio_pmd-to-insert-the-huge-zero-folio.patch
mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special.patch
powerpc-ptdump-rename-struct-pgtable_level-to-struct-ptdump_pglevel.patch
mm-rmap-convert-enum-rmap_level-to-enum-pgtable_level.patch
mm-memory-convert-print_bad_pte-to-print_bad_page_map.patch
mm-memory-convert-print_bad_pte-to-print_bad_page_map-fix.patch
mm-memory-factor-out-common-code-from-vm_normal_page_.patch
mm-introduce-and-use-vm_normal_page_pud.patch
mm-rename-vm_ops-find_special_page-to-vm_ops-find_normal_page.patch
prctl-extend-pr_set_thp_disable-to-optionally-exclude-vm_hugepage.patch
mm-huge_memory-convert-tva_flags-to-enum-tva_type.patch
mm-huge_memory-respect-madv_collapse-with-pr_thp_disable_except_advised.patch
mm-stop-making-sparsemem_vmemmap-user-selectable.patch
arm64-kconfig-drop-superfluous-select-sparsemem_vmemmap.patch
s390-kconfig-drop-superfluous-select-sparsemem_vmemmap.patch
x86-kconfig-drop-superfluous-select-sparsemem_vmemmap.patch
wireguard-selftests-remove-config_sparsemem_vmemmap=y-from-qemu-kernel-config.patch
mm-page_alloc-reject-unreasonable-folio-compound-page-sizes-in-alloc_contig_range_noprof.patch
mm-memremap-reject-unreasonable-folio-compound-page-sizes-in-memremap_pages.patch
mm-hugetlb-check-for-unreasonable-folio-sizes-when-registering-hstate.patch
mm-mm_init-make-memmap_init_compound-look-more-like-prep_compound_page.patch
mm-sanity-check-maximum-folio-size-in-folio_set_order.patch
mm-limit-folio-compound-page-sizes-in-problematic-kernel-configs.patch
mm-simplify-folio_page-and-folio_page_idx.patch
mm-hugetlb-cleanup-hugetlb_folio_init_tail_vmemmap.patch
mm-mm-percpu-km-drop-nth_page-usage-within-single-allocation.patch
fs-hugetlbfs-remove-nth_page-usage-within-folio-in-adjust_range_hwpoison.patch
fs-hugetlbfs-cleanup-folio-in-adjust_range_hwpoison.patch
mm-pagewalk-drop-nth_page-usage-within-folio-in-folio_walk_start.patch
mm-gup-drop-nth_page-usage-within-folio-when-recording-subpages.patch
mm-gup-remove-record_subpages.patch
io_uring-zcrx-remove-nth_page-usage-within-folio.patch
mips-mm-convert-__flush_dcache_pages-to-__flush_dcache_folio_pages.patch
mm-cma-refuse-handing-out-non-contiguous-page-ranges.patch
dma-remap-drop-nth_page-in-dma_common_contiguous_remap.patch
scatterlist-disallow-non-contigous-page-ranges-in-a-single-sg-entry.patch
ata-libata-sff-drop-nth_page-usage-within-sg-entry.patch
drm-i915-gem-drop-nth_page-usage-within-sg-entry.patch
mspro_block-drop-nth_page-usage-within-sg-entry.patch
memstick-drop-nth_page-usage-within-sg-entry.patch
mmc-drop-nth_page-usage-within-sg-entry.patch
scsi-scsi_lib-drop-nth_page-usage-within-sg-entry.patch
scsi-sg-drop-nth_page-usage-within-sg-entry.patch
vfio-pci-drop-nth_page-usage-within-sg-entry.patch
crypto-remove-nth_page-usage-within-sg-entry.patch
mm-gup-drop-nth_page-usage-in-unpin_user_page_range_dirty_lock.patch
kfence-drop-nth_page-usage.patch
block-update-comment-of-struct-bio_vec-regarding-nth_page.patch
mm-remove-nth_page.patch
selftests-mm-split_huge_page_test-fix-occasional-is_backed_by_folio-wrong-results.patch
selftests-mm-split_huge_page_test-cleanups-for-split_pte_mapped_thp-test.patch