linux-kernel - Re: [PATCH 1/3 fix] mm,thp,rmap: subpages

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Date:   Mon, 21 Nov 2022 15:38:12 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        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: Re: [PATCH 1/3 fix] mm,thp,rmap: subpages_mapcount of PTE-mapped
 subpages: fix

On Fri, Nov 18, 2022 at 05:35:05PM -0800, Hugh Dickins wrote:
> 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);

Other option is to drop PageTransHuge() check that you already claim to be
redundant.

Or have else BUG() to catch cases where the helper called with
compound=true on non-THP page.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov