[PATCH 1/3 fix] mm,hugetlb: use folio fields in second tail page: fix

[Hugh Dickins]

Per review comment from Sidhartha: prefix folio's page_1 and page_2 with
double underscore, to underscore that they are fillers for alignment
rather than directly usable members of the union (whereas the first
"struct page page" is important for folio<->page conversions).

Per review comment from Kirill: give folio's _flags_2 and _head_2 a line
of documentation each, though both of them "Do not use" (I think _flags_1
should be enough for now, and shouldn't recommend spilling to _flags_2).

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 include/linux/mm_types.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5d28bbd19e3f..1b8db9b4a7e6 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -275,6 +275,8 @@ struct page {
  * @_subpages_mapcount: Do not use directly, call folio_mapcount().
  * @_pincount: Do not use directly, call folio_maybe_dma_pinned().
  * @_folio_nr_pages: Do not use directly, call folio_nr_pages().
+ * @_flags_2: For alignment.  Do not use.
+ * @_head_2: Points to the folio.  Do not use.
  * @_hugetlb_subpool: Do not use directly, use accessor in hugetlb.h.
  * @_hugetlb_cgroup: Do not use directly, use accessor in hugetlb_cgroup.h.
  * @_hugetlb_cgroup_rsvd: Do not use directly, use accessor in hugetlb_cgroup.h.
@@ -330,7 +332,7 @@ struct folio {
 			unsigned int _folio_nr_pages;
 #endif
 		};
-		struct page page_1;
+		struct page __page_1;
 	};
 	union {
 		struct {
@@ -341,7 +343,7 @@ struct folio {
 			void *_hugetlb_cgroup_rsvd;
 			void *_hugetlb_hwpoison;
 		};
-		struct page page_2;
+		struct page __page_2;
 	};
 };
 
-- 
2.35.3