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    SubjectRe: [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@google.com>
    > Signed-off-by: Hugh Dickins <hughd@google.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

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