Le 24/08/2022 à 08:41, Zhouyi Zhou a écrit :
> Thank Christophe for reviewing my patch
>
> On Wed, Aug 24, 2022 at 2:10 PM Christophe Leroy
> <christophe.leroy@xxxxxxxxxx> 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@xxxxxxxxx>
>>> ---
>>> 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.
I think GCC is smart enough to do the assignment only when necessary,
maybe have a look at the generated assembly in order to confirm.
>>
>>>
>>> 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
Don't spend too much time on that, there are already other people
looking at it, see
https://lore.kernel.org/linux-mm/20220823153055.2517764-1-nathan@xxxxxxxxxx/T/
>
> Thanks
> Zhouyi
>>
>>> pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
>>>
>>> next = hugetlb_entry_end(h, addr, end);
