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Date:   Wed, 9 Nov 2022 18:13:50 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
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        Vlastimil Babka <vbabka@suse.cz>, Peter Xu <peterx@redhat.com>,
        Yang Shi <shy828301@gmail.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Naoya Horiguchi <naoya.horiguchi@linux.dev>,
        Mina Almasry <almasrymina@google.com>,
        James Houghton <jthoughton@google.com>,
        Zach O'Keefe <zokeefe@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH 1/3 fix] mm,hugetlb: use folio fields in second tail page:
 fix
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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@google.com>
---
 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