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Date:   Sun, 23 Jul 2023 15:26:02 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
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Subject: [PATCH v3 04/13 fix] powerpc: assert_pte_locked() use pte_offset_map_nolock():
 fix
In-Reply-To: <20230721131341.w5abuxcbohofpzwa@patel>
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References: <7cd843a9-aa80-14f-5eb2-33427363c20@google.com> <e8d56c95-c132-a82e-5f5f-7bb1b738b057@google.com> <87msztbiy8.fsf@linux.ibm.com> <392f311f-83ac-a5a2-d16e-2c7736d1b577@google.com> <6762c880-6d2b-233f-6786-7ad5b0472dc7@linux.ibm.com>
 <20230721131341.w5abuxcbohofpzwa@patel>
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Aneesh points out that assert_pte_locked() still needs the pmd_none()
check, to stop crashing in khugepaged: restore that comment and check.

Andrew, when merging with original commit, please edit its 1st para to:

Instead of pte_lockptr(), use the recently added pte_offset_map_nolock()
in assert_pte_locked().  BUG if pte_offset_map_nolock() fails.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 arch/powerpc/mm/pgtable.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index 16b061af86d7..a3dcdb2d5b4b 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -323,6 +323,14 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
 	pud = pud_offset(p4d, addr);
 	BUG_ON(pud_none(*pud));
 	pmd = pmd_offset(pud, addr);
+	/*
+	 * khugepaged to collapse normal pages to hugepage, first set
+	 * pmd to none to force page fault/gup to take mmap_lock. After
+	 * pmd is set to none, we do a pte_clear which does this assertion
+	 * so if we find pmd none, return.
+	 */
+	if (pmd_none(*pmd))
+		return;
 	pte = pte_offset_map_nolock(mm, pmd, addr, &ptl);
 	BUG_ON(!pte);
 	assert_spin_locked(ptl);
-- 
2.35.3