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 <[email protected]>
---
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;
for (; addr < end; addr = next) {
- int err;
pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
next = hugetlb_entry_end(h, addr, end);
--
2.34.1
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
> <[email protected]> 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 <[email protected]>
>>> ---
>>> 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/[email protected]/T/
>
> Thanks
> Zhouyi
>>
>>> pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
>>>
>>> next = hugetlb_entry_end(h, addr, end);
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 <[email protected]>
> ---
> 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 ?
>
> 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.
> pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
>
> next = hugetlb_entry_end(h, addr, end);
Thank Christophe for reviewing my patch
On Wed, Aug 24, 2022 at 2:10 PM Christophe Leroy
<[email protected]> 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 <[email protected]>
> > ---
> > 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);
On Wed, Aug 24, 2022 at 2:49 PM Christophe Leroy
<[email protected]> wrote:
>
>
>
> 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
> > <[email protected]> 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 <[email protected]>
> >>> ---
> >>> 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.
Thank Christophe for your guidance, I disassembled the GCC generated
code, it optimized away int err = 0 on both X86 and PowerPC platforms.
The disassembled instructions are quite long, so I prefer not to paste
in my email unless someone asks me to do so ;-)
>
> >>
> >>>
> >>> 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
>
Thank Christophe for your reminder, I only subscribed to the RCU
mailing list currently, so I submitted duplicated work. I should be
more devoted to the Linux community. I am sorry.
> Don't spend too much time on that, there are already other people
> looking at it, see
> https://lore.kernel.org/linux-mm/[email protected]/T/
>
> >
> > Thanks
> > Zhouyi
Many thanks
Zhouyi
> >>
> >>> pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz);
> >>>
> >>> next = hugetlb_entry_end(h, addr, end);