[RFC v1 02/10] powerpc: book3s64: Fix unmap race with PMD THP migration entry

[Ritesh Harjani (IBM)]

The following race is possible with migration swap entries or
device-private THP entries. e.g. when move_pages is called on a PMD THP
page, then there maybe an intermediate state, where PMD entry acts as
a migration swap entry (pmd_present() is true). Then if an munmap
happens at the same time, then this VM_BUG_ON() can happen in
pmdp_huge_get_and_clear_full().

This patch fixes that.

Thread A: move_pages() syscall
  add_folio_for_migration()
    mmap_read_lock(mm)
    folio_isolate_lru(folio)
    mmap_read_unlock(mm)

  do_move_pages_to_node()
    migrate_pages()
      try_to_migrate_one()
        spin_lock(ptl)
        set_pmd_migration_entry()
          pmdp_invalidate()     # PMD: _PAGE_INVALID | _PAGE_PTE | pfn
          set_pmd_at()          # PMD: migration swap entry (pmd_present=0)
        spin_unlock(ptl)
        [page copy phase]       # <--- RACE WINDOW -->

Thread B: munmap()
  mmap_write_downgrade(mm)
  unmap_vmas() -> zap_pmd_range()
    zap_huge_pmd()
      __pmd_trans_huge_lock()
        pmd_is_huge():          # !pmd_present && !pmd_none -> TRUE (swap entry)
        pmd_lock() -> 		# spin_lock(ptl), waits for Thread A to release ptl
      pmdp_huge_get_and_clear_full()
        VM_BUG_ON(!pmd_present(*pmdp))  # HITS!

[  287.738700][ T1867] ------------[ cut here ]------------
[  287.743843][ T1867] kernel BUG at arch/powerpc/mm/book3s64/pgtable.c:187!
cpu 0x0: Vector: 700 (Program Check) at [c00000044037f4f0]
    pc: c000000000094ca4: pmdp_huge_get_and_clear_full+0x6c/0x23c
    lr: c000000000645dec: zap_huge_pmd+0xb0/0x868
    sp: c00000044037f790
   msr: 800000000282b033
  current = 0xc0000004032c1a00
  paca    = 0xc000000004fe0000   irqmask: 0x03   irq_happened: 0x09
    pid   = 1867, comm = a.out
kernel BUG at :187!
Linux version 6.19.0-12136-g14360d4f917c-dirty (powerpc64le-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40) #27 SMP PREEMPT Sun Feb 22 10:38:56 IST 2026
enter ? for help
[link register   ] c000000000645dec zap_huge_pmd+0xb0/0x868
[c00000044037f790] c00000044037f7d0 (unreliable)
[c00000044037f7d0] c000000000645dcc zap_huge_pmd+0x90/0x868
[c00000044037f840] c0000000005724cc unmap_page_range+0x176c/0x1f40
[c00000044037fa00] c000000000572ea0 unmap_vmas+0xb0/0x1d8
[c00000044037fa90] c0000000005af254 unmap_region+0xb4/0x128
[c00000044037fb50] c0000000005af400 vms_complete_munmap_vmas+0x138/0x310
[c00000044037fbe0] c0000000005b0f1c do_vmi_align_munmap+0x1ec/0x238
[c00000044037fd30] c0000000005b3688 __vm_munmap+0x170/0x1f8
[c00000044037fdf0] c000000000587f74 sys_munmap+0x2c/0x40
[c00000044037fe10] c000000000032668 system_call_exception+0x128/0x350
[c00000044037fe50] c00000000000d05c system_call_vectored_common+0x15c/0x2ec
---- Exception: 3000 (System Call Vectored) at 0000000010064a2c
SP (7fff9b1ee9c0) is in userspace
0:mon> zh

Fixes: 75358ea359e7c ("powerpc/mm/book3s64: Fix MADV_DONTNEED and parallel page fault race")
Reported-by: Pavithra Prakash <pavrampu at linux.vnet.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list at gmail.com>
---
 arch/powerpc/mm/book3s64/pgtable.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 4b09c04654a8..359092001670 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -210,8 +210,23 @@ pmd_t pmdp_huge_get_and_clear_full(struct vm_area_struct *vma,
 {
 	pmd_t pmd;
 	VM_BUG_ON(addr & ~HPAGE_PMD_MASK);
-	VM_BUG_ON((pmd_present(*pmdp) && !pmd_trans_huge(*pmdp)) ||
-		   !pmd_present(*pmdp));
+	VM_BUG_ON((pmd_present(*pmdp) && !pmd_trans_huge(*pmdp)));
+
+	if (!pmd_present(*pmdp)) {
+		/*
+		 * Non-present PMDs can be migration entries or device-private
+		 * THP entries. Since these are non-present, so there is no TLB
+		 * backing. This happens when the address space is being
+		 * unmapped zap_huge_pmd(), and we encounter non-present pmds.
+		 * So it is safe to just clear the PMDs here. zap_huge_pmd(),
+		 * will take care of withdraw of the deposited table.
+		 */
+		pmd = pmdp_get(pmdp);
+		pmd_clear(pmdp);
+		page_table_check_pmd_clear(vma->vm_mm, addr, pmd);
+		return pmd;
+	}
+
 	pmd = pmdp_huge_get_and_clear(vma->vm_mm, addr, pmdp);
 	/*
 	 * if it not a fullmm flush, then we can possibly end up converting
--
2.53.0