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

[Kirill A. Shutemov]

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@xxxxxxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>  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