Re: [PATCH v2] mm: introduce arch_has_hw_nonleaf_pmd_young()

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On 23.11.22 07:45, Juergen Gross wrote:
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_nonleaf_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@xxxxxxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Acked-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Tested-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
---
V2:
- correct function name in commit message to match patch
---
  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);

Acked-by: David Hildenbrand <david@xxxxxxxxxx> # core changes

--
Thanks,

David / dhildenb





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