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From:   Juergen Gross <jgross@suse.com>
To:     linux-kernel@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org
Cc:     yuzhao@google.com, Juergen Gross <jgross@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sander Eikelenboom <linux@eikelenboom.it>
Subject: [PATCH] mm: introduce arch_has_hw_pmd_young()
Date:   Mon, 21 Nov 2022 10:32:45 +0100
Message-Id: <20221121093245.4587-1-jgross@suse.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

When running as a Xen PV guests commit eed9a328aa1a ("mm: x86: add
CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG") can cause a protection violation
in pmdp_test_and_clear_young():

 BUG: unable to handle page fault for address: ffff8880083374d0
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0003) - permissions violation
 PGD 3026067 P4D 3026067 PUD 3027067 PMD 7fee5067 PTE 8010000008337065
 Oops: 0003 [#1] PREEMPT SMP NOPTI
 CPU: 7 PID: 158 Comm: kswapd0 Not tainted 6.1.0-rc5-20221118-doflr+ #1
 RIP: e030:pmdp_test_and_clear_young+0x25/0x40

This happens because the Xen hypervisor can't emulate direct writes to
page table entries other than PTEs.

This can easily be fixed by introducing arch_has_hw_pmd_young()
similar to arch_has_hw_pte_young() and test that instead of
CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG.

Fixes: eed9a328aa1a ("mm: x86: add CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG")
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/include/asm/pgtable.h |  8 ++++++++
 include/linux/pgtable.h        | 11 +++++++++++
 mm/vmscan.c                    | 10 +++++-----
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 5059799bebe3..c567a6ed17ce 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1438,6 +1438,14 @@ static inline bool arch_has_hw_pte_young(void)
 	return true;
 }
 
+#ifdef CONFIG_XEN_PV
+#define arch_has_hw_nonleaf_pmd_young arch_has_hw_nonleaf_pmd_young
+static inline bool arch_has_hw_nonleaf_pmd_young(void)
+{
+	return !cpu_feature_enabled(X86_FEATURE_XENPV);
+}
+#endif
+
 #ifdef CONFIG_PAGE_TABLE_CHECK
 static inline bool pte_user_accessible_page(pte_t pte)
 {
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index a108b60a6962..58fc7e2d9575 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -260,6 +260,17 @@ static inline int pmdp_clear_flush_young(struct vm_area_struct *vma,
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 #endif
 
+#ifndef arch_has_hw_nonleaf_pmd_young
+/*
+ * Return whether the accessed bit in non-leaf PMD entries is supported on the
+ * local CPU.
+ */
+static inline bool arch_has_hw_nonleaf_pmd_young(void)
+{
+	return IS_ENABLED(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG);
+}
+#endif
+
 #ifndef arch_has_hw_pte_young
 /*
  * Return whether the accessed bit is supported on the local CPU.
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 04d8b88e5216..a04ac3b18326 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3975,7 +3975,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long next, struct vm_area
 			goto next;
 
 		if (!pmd_trans_huge(pmd[i])) {
-			if (IS_ENABLED(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG) &&
+			if (arch_has_hw_nonleaf_pmd_young() &&
 			    get_cap(LRU_GEN_NONLEAF_YOUNG))
 				pmdp_test_and_clear_young(vma, addr, pmd + i);
 			goto next;
@@ -4073,14 +4073,14 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 #endif
 		walk->mm_stats[MM_NONLEAF_TOTAL]++;
 
-#ifdef CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG
-		if (get_cap(LRU_GEN_NONLEAF_YOUNG)) {
+		if (arch_has_hw_nonleaf_pmd_young() &&
+		    get_cap(LRU_GEN_NONLEAF_YOUNG)) {
 			if (!pmd_young(val))
 				continue;
 
 			walk_pmd_range_locked(pud, addr, vma, args, bitmap, &pos);
 		}
-#endif
+
 		if (!walk->force_scan && !test_bloom_filter(walk->lruvec, walk->max_seq, pmd + i))
 			continue;
 
@@ -5354,7 +5354,7 @@ static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, c
 	if (arch_has_hw_pte_young() && get_cap(LRU_GEN_MM_WALK))
 		caps |= BIT(LRU_GEN_MM_WALK);
 
-	if (IS_ENABLED(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG) && get_cap(LRU_GEN_NONLEAF_YOUNG))
+	if (arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG))
 		caps |= BIT(LRU_GEN_NONLEAF_YOUNG);
 
 	return snprintf(buf, PAGE_SIZE, "0x%04x\n", caps);
-- 
2.35.3