[PATCH 24/46] rmap: update hugetlb lock comment for HGM

[James Houghton]

The VMA lock is used to prevent high-granularity HugeTLB mappings from
being collapsed while other threads are doing high-granularity page
table walks.

Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
---
 include/linux/hugetlb.h | 12 ++++++++++++
 mm/rmap.c               |  3 ++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index b7cf45535d64..daf993fdbc38 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -156,6 +156,18 @@ struct file_region {
 #endif
 };
 
+/*
+ * The HugeTLB VMA lock is used to synchronize HugeTLB page table walks.
+ * Right now, it is only used for VM_SHARED mappings.
+ * - The read lock is held when we want to stabilize mappings (prevent PMD
+ *   unsharing or MADV_COLLAPSE for high-granularity mappings).
+ * - The write lock is held when we want to free mappings (PMD unsharing and
+ *   MADV_COLLAPSE for high-granularity mappings).
+ *
+ * Note: For PMD unsharing and MADV_COLLAPSE, the i_mmap_rwsem is held for
+ * writing as well, so page table walkers will also be safe if they hold
+ * i_mmap_rwsem for at least reading. See hugetlb_walk() for more information.
+ */
 struct hugetlb_vma_lock {
 	struct kref refs;
 	struct rw_semaphore rw_sema;
diff --git a/mm/rmap.c b/mm/rmap.c
index ff7e6c770b0a..076ea77010e5 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -47,7 +47,8 @@
  *
  * hugetlbfs PageHuge() take locks in this order:
  *   hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
- *     vma_lock (hugetlb specific lock for pmd_sharing)
+ *     vma_lock (hugetlb specific lock for pmd_sharing and high-granularity
+ *               mapping)
  *       mapping->i_mmap_rwsem (also used for hugetlb pmd sharing)
  *         page->flags PG_locked (lock_page)
  */
-- 
2.39.0.314.g84b9a713c41-goog