linux-kernel - [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <v5ivpub6z2n2uyemlnxgbilzs52ep4lrary7lm7o6axxoneb75@yfacfl5rkzeh>
Date: Mon, 15 Sep 2025 14:52:53 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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>, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on
 SCAN_PMD_NULL

From: Kiryl Shutsemau <kas@...nel.org>

MADV_COLLAPSE on a file mapping behaves inconsistently depending on if
PMD page table is installed or not.

Consider following example:

	p = mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
		 MAP_SHARED, fd, 0);
	err = madvise(p, 2UL << 20, MADV_COLLAPSE);

fd is a populated tmpfs file.

The result depends on the address that the kernel returns on mmap().
If it is located in an existing PMD table, the madvise() will succeed.
However, if the table does not exist, it will fail with -EINVAL.

This occurs because find_pmd_or_thp_or_none() returns SCAN_PMD_NULL when
a page table is missing, which causes collapse_pte_mapped_thp() to fail.

SCAN_PMD_NULL and SCAN_PMD_NONE should be treated the same in
collapse_pte_mapped_thp(): install the PMD leaf entry and allocate page
tables as needed.

Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
---

v2:
 - Modify set_huge_pmd() instead of introducing install_huge_pmd();

---
 mm/khugepaged.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b486c1d19b2d..986718599355 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1472,15 +1472,32 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
 static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
 			pmd_t *pmdp, struct folio *folio, struct page *page)
 {
+	struct mm_struct *mm = vma->vm_mm;
 	struct vm_fault vmf = {
 		.vma = vma,
 		.address = addr,
 		.flags = 0,
-		.pmd = pmdp,
 	};
+	pgd_t *pgdp;
+	p4d_t *p4dp;
+	pud_t *pudp;
 
 	mmap_assert_locked(vma->vm_mm);
 
+	if (!pmdp) {
+		pgdp = pgd_offset(mm, addr);
+		p4dp = p4d_alloc(mm, pgdp, addr);
+		if (!p4dp)
+			return SCAN_FAIL;
+		pudp = pud_alloc(mm, p4dp, addr);
+		if (!pudp)
+			return SCAN_FAIL;
+		pmdp = pmd_alloc(mm, pudp, addr);
+		if (!pmdp)
+			return SCAN_FAIL;
+	}
+
+	vmf.pmd = pmdp;
 	if (do_set_pmd(&vmf, folio, page))
 		return SCAN_FAIL;
 
@@ -1556,6 +1573,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 	switch (result) {
 	case SCAN_SUCCEED:
 		break;
+	case SCAN_PMD_NULL:
 	case SCAN_PMD_NONE:
 		/*
 		 * All pte entries have been removed and pmd cleared.
-- 
2.50.1