linux-kernel - [PATCH v3 04/13] powerpc: assert_pte_locked() use pte_offset_map

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Message-ID: <e8d56c95-c132-a82e-5f5f-7bb1b738b057@google.com>
Date:   Tue, 11 Jul 2023 21:34:25 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
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        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
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        Peter Zijlstra <peterz@...radead.org>,
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        SeongJae Park <sj@...nel.org>,
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        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v3 04/13] powerpc: assert_pte_locked() use
 pte_offset_map_nolock()

Instead of pte_lockptr(), use the recently added pte_offset_map_nolock()
in assert_pte_locked().  BUG if pte_offset_map_nolock() fails: this is
stricter than the previous implementation, which skipped when pmd_none()
(with a comment on khugepaged collapse transitions): but wouldn't we want
to know, if an assert_pte_locked() caller can be racing such transitions?

This mod might cause new crashes: which either expose my ignorance, or
indicate issues to be fixed, or limit the usage of assert_pte_locked().

Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
 arch/powerpc/mm/pgtable.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index cb2dcdb18f8e..16b061af86d7 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -311,6 +311,8 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
 	p4d_t *p4d;
 	pud_t *pud;
 	pmd_t *pmd;
+	pte_t *pte;
+	spinlock_t *ptl;
 
 	if (mm == &init_mm)
 		return;
@@ -321,16 +323,10 @@ 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;
-	BUG_ON(!pmd_present(*pmd));
-	assert_spin_locked(pte_lockptr(mm, pmd));
+	pte = pte_offset_map_nolock(mm, pmd, addr, &ptl);
+	BUG_ON(!pte);
+	assert_spin_locked(ptl);
+	pte_unmap(pte);
 }
 #endif /* CONFIG_DEBUG_VM */
 
-- 
2.35.3