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    SubjectRe: [PATCH linux-next] mm: fix used but uninitialized variable
    Thank Christophe for reviewing my patch

    On Wed, Aug 24, 2022 at 2:10 PM Christophe Leroy
    <christophe.leroy@csgroup.eu> wrote:
    >
    >
    >
    > Le 24/08/2022 à 07:56, Zhouyi Zhou a écrit :
    > > In function walk_hugetlb_range, the local variable err may
    > > be used uninitialzed when:
    > > ops->pte_hole in side of "else if (ops->pte_hole)" is false.
    > >
    > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
    > > ---
    > > Dear mm Developers:
    > >
    > > When I build kernel using "make CC=clang-14"
    > > the compiler complains following:
    > >
    > > CC mm/pagewalk.o
    > > mm/pagewalk.c:318:12: error: variable 'err' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    > > else if (ops->pte_hole)
    > > ^~~~~~~~~~~~~
    > > mm/pagewalk.c:321:7: note: uninitialized use occurs here
    > > if (err)
    > > ^~~
    > > mm/pagewalk.c:318:8: note: remove the 'if' if its condition is always true
    > > else if (ops->pte_hole)
    > > ^~~~~~~~~~~~~~~~~~
    > > mm/pagewalk.c:311:10: note: initialize the variable 'err' to silence this warning
    > > int err;
    > > ^
    > > = 0
    > > 1 error generated.
    > > make[1]: *** [scripts/Makefile.build:250: mm/pagewalk.o] Error 1
    > > make: *** [Makefile:2006: mm] Error 2
    > >
    > > I initialize that variable outside of the for loop because we can assign 0 to err
    > > only once in this function.
    > >
    > > After my fix, I can compile the kernel.
    > >
    > > Many Thanks
    > > Zhouyi
    > > --
    > > mm/pagewalk.c | 2 +-
    > > 1 file changed, 1 insertion(+), 1 deletion(-)
    > >
    > > diff --git a/mm/pagewalk.c b/mm/pagewalk.c
    > > index 54b2a1beeeb3..b6eb330e8ecd 100644
    > > --- a/mm/pagewalk.c
    > > +++ b/mm/pagewalk.c
    > > @@ -306,9 +306,9 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
    > > unsigned long hmask = huge_page_mask(h);
    > > unsigned long sz = huge_page_size(h);
    > > const struct mm_walk_ops *ops = walk->ops;
    > > + int err = 0;
    >
    > Why do you move it back outside of the for loop allthough it is
    > exclusively used inside the loop ?
    I move it outside of the for loop for performance consideration. Because
    if we initialize err inside, there will be an assignment statement
    every iteration.
    >
    > >
    > > for (; addr < end; addr = next) {
    > > - int err;
    >
    > Another solution would be to add an explicit else, setting err = 0 in
    > the if/else if sequence.
    Thank Christophe for your valuable advice, I am going to prepare a 2nd version

    Thanks
    Zhouyi
    >
    > > pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
    > >
    > > next = hugetlb_entry_end(h, addr, end);

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