linux-kernel - [PATCH 1/3 fix] mm,thp,rmap: subpages_mapcount of PTE-mapped subpages: fix

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Date:   Fri, 18 Nov 2022 17:35:05 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Yu Zhao <yuzhao@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Mina Almasry <almasrymina@...gle.com>,
        James Houghton <jthoughton@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 1/3 fix] mm,thp,rmap: subpages_mapcount of PTE-mapped subpages:
 fix

Yu Zhao reports compiler warning in page_add_anon_rmap():

mm/rmap.c:1236:13: warning: variable 'first' is used uninitialized
whenever 'if' condition is false [-Wsometimes-uninitialized]
        } else if (PageTransHuge(page)) {
                   ^~~~~~~~~~~~~~~~~~~
mm/rmap.c:1248:18: note: uninitialized use occurs here
        VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
                        ^~~~~

We do need to fix that, even though it's only uninitialized in an
impossible condition: I've chosen to initialize "first" true, to
minimize the BUGs it might then hit; but you could just as well
choose to initialize it false, to maximize the BUGs it might hit.

Reported-by: Yu Zhao <yuzhao@...gle.com>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
 mm/rmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 66be8cae640f..25b720d5ba17 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1281,7 +1281,7 @@ void page_add_anon_rmap(struct page *page,
 	struct compound_mapcounts mapcounts;
 	int nr = 0, nr_pmdmapped = 0;
 	bool compound = flags & RMAP_COMPOUND;
-	bool first;
+	bool first = true;
 
 	if (unlikely(PageKsm(page)))
 		lock_page_memcg(page);
-- 
2.35.3