include/linux/hugetlb.h: add synchronization information for new hugetlb specific flags

Add comments, no functional change.

Link: https://lkml.kernel.org/r/62a80585-2a73-10cc-4a2d-5721540d4ad2@oracle.comSigned-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/hugetlb.h

@@ -480,14 +480,24 @@ unsigned long hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
  * HPG_restore_reserve - Set when a hugetlb page consumes a reservation at
  *	allocation time.  Cleared when page is fully instantiated.  Free
  *	routine checks flag to restore a reservation on error paths.
+ *	Synchronization:  Examined or modified by code that knows it has
+ *	the only reference to page.  i.e. After allocation but before use
+ *	or when the page is being freed.
  * HPG_migratable  - Set after a newly allocated page is added to the page
  *	cache and/or page tables.  Indicates the page is a candidate for
  *	migration.
+ *	Synchronization:  Initially set after new page allocation with no
+ *	locking.  When examined and modified during migration processing
+ *	(isolate, migrate, putback) the hugetlb_lock is held.
  * HPG_temporary - - Set on a page that is temporarily allocated from the buddy
  *	allocator.  Typically used for migration target pages when no pages
  *	are available in the pool.  The hugetlb free page path will
  *	immediately free pages with this flag set to the buddy allocator.
+ *	Synchronization: Can be set after huge page allocation from buddy when
+ *	code knows it has only reference.  All other examinations and
+ *	modifications require hugetlb_lock.
  * HPG_freed - Set when page is on the free lists.
+ *	Synchronization: hugetlb_lock held for examination and modification.
  */
 enum hugetlb_page_flags {
 	HPG_restore_reserve = 0,