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Message-ID: <20250903070253.34556-2-david@redhat.com>
Date: Wed, 3 Sep 2025 09:02:52 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
linux-kselftest@...r.kernel.org,
David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Zi Yan <ziy@...dia.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Nico Pache <npache@...hat.com>,
Ryan Roberts <ryan.roberts@....com>,
Dev Jain <dev.jain@....com>,
Barry Song <baohua@...nel.org>,
Wei Yang <richard.weiyang@...il.com>
Subject: [PATCH v2 1/2] selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results
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@...dia.com>
Reviewed-by: Wei Yang <richard.weiyang@...il.com>
Signed-off-by: David Hildenbrand <david@...hat.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
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