mm: pagewalk: assert write mmap lock only for walking the user page tables

The 8782fb61 ("mm: pagewalk: Fix race between unmap and page walker")
introduces an assertion to walk_page_range_novma() to make all the users
of page table walker is safe.  However, the race only exists for walking
the user page tables.  And it is ridiculous to hold a particular user mmap
write lock against the changes of the kernel page tables.  So only assert
at least mmap read lock when walking the kernel page tables.  And some
users matching this case could downgrade to a mmap read lock to relief the
contention of mmap lock of init_mm, it will be nicer in hugetlb (only
holding mmap read lock) in the next patch.

Link: https://lkml.kernel.org/r/20231127084645.27017-2-songmuchun@bytedance.comSigned-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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);
 }