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-rw-r--r--mm/pagewalk.c29
1 files changed, 28 insertions, 1 deletions
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index b7d7e4fcfa..f46c80b18c 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -539,6 +539,11 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
* not backed by VMAs. Because 'unusual' entries may be walked this function
* will also not lock the PTEs for the pte_entry() callback. This is useful for
* walking the kernel pages tables or page tables for firmware.
+ *
+ * Note: Be careful to walk the kernel pages tables, the caller may be need to
+ * take other effective approache (mmap lock may be insufficient) to prevent
+ * the intermediate kernel page tables belonging to the specified address range
+ * from being freed (e.g. memory hot-remove).
*/
int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
unsigned long end, const struct mm_walk_ops *ops,
@@ -556,7 +561,29 @@ int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
if (start >= end || !walk.mm)
return -EINVAL;
- mmap_assert_write_locked(walk.mm);
+ /*
+ * 1) For walking the user virtual address space:
+ *
+ * The mmap lock protects the page walker from changes to the page
+ * tables during the walk. However a read lock is insufficient to
+ * protect those areas which don't have a VMA as munmap() detaches
+ * the VMAs before downgrading to a read lock and actually tearing
+ * down PTEs/page tables. In which case, the mmap write lock should
+ * be hold.
+ *
+ * 2) For walking the kernel virtual address space:
+ *
+ * The kernel intermediate page tables usually do not be freed, so
+ * the mmap map read lock is sufficient. But there are some exceptions.
+ * E.g. memory hot-remove. In which case, the mmap lock is insufficient
+ * to prevent the intermediate kernel pages tables belonging to the
+ * specified address range from being freed. The caller should take
+ * other actions to prevent this race.
+ */
+ if (mm == &init_mm)
+ mmap_assert_locked(walk.mm);
+ else
+ mmap_assert_write_locked(walk.mm);
return walk_pgd_range(start, end, &walk);
}