selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results

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>

diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index 10ae65ea032f612157a51e1eafbe6ac0855c505a..72d6d8bb329edd4c1b7a0a6a25ac4f0c13a23d74 100644 (file)
--- 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;