Re: [PATCH] mm: remove a redundant condition in the for loop

[Michal Hocko]

On Mon 19-06-17 21:05:29, Rasmus Villemoes wrote:
> On Mon, Jun 19 2017, Vlastimil Babka  wrote:
> 
> > On 06/19/2017 03:54 PM, Hao Lee wrote:
> >> The variable current_order decreases from MAX_ORDER-1 to order, so the
> >> condition current_order <= MAX_ORDER-1 is always true.
> >> 
> >> Signed-off-by: Hao Lee 
> >
> > Sounds right.
> >
> > Acked-by: Vlastimil Babka 
> 
> current_order and order are both unsigned, and if order==0,
> current_order >= order is always true, and we may decrement
> current_order past 0 making it UINT_MAX... A comment would be in order,
> though.

Yes, not the first time this has been brought up
https://lkml.org/lkml/2016/6/20/493. I guess a comment is long overdue.
Or just get rid of the unsigned trap which would be probably more clean.
 
> >> ---
> >>  mm/page_alloc.c | 5 ++---
> >>  1 file changed, 2 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> index 2302f25..9120c2b 100644
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int 
> >> order, int start_migratetype)
> >>bool can_steal;
> >>  
> >>/* Find the largest possible block of pages in the other list */
> >> -  for (current_order = MAX_ORDER-1;
> >> -  current_order >= order && current_order <= 
> >> MAX_ORDER-1;
> >> -  --current_order) {
> >> +  for (current_order = MAX_ORDER-1; current_order >= order;
> >> +  --current_order) {
> >>area = &(zone->free_area[current_order]);
> >>fallback_mt = find_suitable_fallback(area, current_order,
> >>start_migratetype, false, _steal);
> >> 

-- 
Michal Hocko
SUSE Labs

Re: [PATCH] mm: remove a redundant condition in the for loop

[Michal Hocko]

On Mon 19-06-17 21:05:29, Rasmus Villemoes wrote:
> On Mon, Jun 19 2017, Vlastimil Babka  wrote:
> 
> > On 06/19/2017 03:54 PM, Hao Lee wrote:
> >> The variable current_order decreases from MAX_ORDER-1 to order, so the
> >> condition current_order <= MAX_ORDER-1 is always true.
> >> 
> >> Signed-off-by: Hao Lee 
> >
> > Sounds right.
> >
> > Acked-by: Vlastimil Babka 
> 
> current_order and order are both unsigned, and if order==0,
> current_order >= order is always true, and we may decrement
> current_order past 0 making it UINT_MAX... A comment would be in order,
> though.

Yes, not the first time this has been brought up
https://lkml.org/lkml/2016/6/20/493. I guess a comment is long overdue.
Or just get rid of the unsigned trap which would be probably more clean.
 
> >> ---
> >>  mm/page_alloc.c | 5 ++---
> >>  1 file changed, 2 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> index 2302f25..9120c2b 100644
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int 
> >> order, int start_migratetype)
> >>bool can_steal;
> >>  
> >>/* Find the largest possible block of pages in the other list */
> >> -  for (current_order = MAX_ORDER-1;
> >> -  current_order >= order && current_order <= 
> >> MAX_ORDER-1;
> >> -  --current_order) {
> >> +  for (current_order = MAX_ORDER-1; current_order >= order;
> >> +  --current_order) {
> >>area = &(zone->free_area[current_order]);
> >>fallback_mt = find_suitable_fallback(area, current_order,
> >>start_migratetype, false, _steal);
> >> 

-- 
Michal Hocko
SUSE Labs

Re: [PATCH] mm: remove a redundant condition in the for loop

[Hao Lee]

On Tue, Jun 20, 2017 at 3:05 AM, Rasmus Villemoes
 wrote:
> On Mon, Jun 19 2017, Vlastimil Babka  wrote:
>
>> On 06/19/2017 03:54 PM, Hao Lee wrote:
>>> The variable current_order decreases from MAX_ORDER-1 to order, so the
>>> condition current_order <= MAX_ORDER-1 is always true.
>>>
>>> Signed-off-by: Hao Lee 
>>
>> Sounds right.
>>
>> Acked-by: Vlastimil Babka 
>
> current_order and order are both unsigned, and if order==0,
> current_order >= order is always true, and we may decrement
> current_order past 0 making it UINT_MAX... A comment would be in order,
> though.

Thanks, I didn't notice unsigned subtraction. Sorry about that.

Re: [PATCH] mm: remove a redundant condition in the for loop

[Hao Lee]

On Tue, Jun 20, 2017 at 3:05 AM, Rasmus Villemoes
 wrote:
> On Mon, Jun 19 2017, Vlastimil Babka  wrote:
>
>> On 06/19/2017 03:54 PM, Hao Lee wrote:
>>> The variable current_order decreases from MAX_ORDER-1 to order, so the
>>> condition current_order <= MAX_ORDER-1 is always true.
>>>
>>> Signed-off-by: Hao Lee 
>>
>> Sounds right.
>>
>> Acked-by: Vlastimil Babka 
>
> current_order and order are both unsigned, and if order==0,
> current_order >= order is always true, and we may decrement
> current_order past 0 making it UINT_MAX... A comment would be in order,
> though.

Thanks, I didn't notice unsigned subtraction. Sorry about that.

Re: [PATCH] mm: remove a redundant condition in the for loop

[Vlastimil Babka]

On 06/19/2017 09:05 PM, Rasmus Villemoes wrote:
> On Mon, Jun 19 2017, Vlastimil Babka  wrote:
> 
>> On 06/19/2017 03:54 PM, Hao Lee wrote:
>>> The variable current_order decreases from MAX_ORDER-1 to order, so the
>>> condition current_order <= MAX_ORDER-1 is always true.
>>>
>>> Signed-off-by: Hao Lee 
>>
>> Sounds right.
>>
>> Acked-by: Vlastimil Babka 
> 
> current_order and order are both unsigned, and if order==0,
> current_order >= order is always true, and we may decrement
> current_order past 0 making it UINT_MAX... A comment would be in order,
> though.

Doh, right. Thanks.

Re: [PATCH] mm: remove a redundant condition in the for loop

[Vlastimil Babka]

On 06/19/2017 09:05 PM, Rasmus Villemoes wrote:
> On Mon, Jun 19 2017, Vlastimil Babka  wrote:
> 
>> On 06/19/2017 03:54 PM, Hao Lee wrote:
>>> The variable current_order decreases from MAX_ORDER-1 to order, so the
>>> condition current_order <= MAX_ORDER-1 is always true.
>>>
>>> Signed-off-by: Hao Lee 
>>
>> Sounds right.
>>
>> Acked-by: Vlastimil Babka 
> 
> current_order and order are both unsigned, and if order==0,
> current_order >= order is always true, and we may decrement
> current_order past 0 making it UINT_MAX... A comment would be in order,
> though.

Doh, right. Thanks.

Re: [PATCH] mm: remove a redundant condition in the for loop

[Rasmus Villemoes]

On Mon, Jun 19 2017, Vlastimil Babka  wrote:

> On 06/19/2017 03:54 PM, Hao Lee wrote:
>> The variable current_order decreases from MAX_ORDER-1 to order, so the
>> condition current_order <= MAX_ORDER-1 is always true.
>> 
>> Signed-off-by: Hao Lee 
>
> Sounds right.
>
> Acked-by: Vlastimil Babka 

current_order and order are both unsigned, and if order==0,
current_order >= order is always true, and we may decrement
current_order past 0 making it UINT_MAX... A comment would be in order,
though.

>> ---
>>  mm/page_alloc.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2302f25..9120c2b 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int 
>> order, int start_migratetype)
>>  bool can_steal;
>>  
>>  /* Find the largest possible block of pages in the other list */
>> -for (current_order = MAX_ORDER-1;
>> -current_order >= order && current_order <= 
>> MAX_ORDER-1;
>> ---current_order) {
>> +for (current_order = MAX_ORDER-1; current_order >= order;
>> +--current_order) {
>>  area = &(zone->free_area[current_order]);
>>  fallback_mt = find_suitable_fallback(area, current_order,
>>  start_migratetype, false, _steal);
>> 

Re: [PATCH] mm: remove a redundant condition in the for loop

[Rasmus Villemoes]

On Mon, Jun 19 2017, Vlastimil Babka  wrote:

> On 06/19/2017 03:54 PM, Hao Lee wrote:
>> The variable current_order decreases from MAX_ORDER-1 to order, so the
>> condition current_order <= MAX_ORDER-1 is always true.
>> 
>> Signed-off-by: Hao Lee 
>
> Sounds right.
>
> Acked-by: Vlastimil Babka 

current_order and order are both unsigned, and if order==0,
current_order >= order is always true, and we may decrement
current_order past 0 making it UINT_MAX... A comment would be in order,
though.

>> ---
>>  mm/page_alloc.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2302f25..9120c2b 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int 
>> order, int start_migratetype)
>>  bool can_steal;
>>  
>>  /* Find the largest possible block of pages in the other list */
>> -for (current_order = MAX_ORDER-1;
>> -current_order >= order && current_order <= 
>> MAX_ORDER-1;
>> ---current_order) {
>> +for (current_order = MAX_ORDER-1; current_order >= order;
>> +--current_order) {
>>  area = &(zone->free_area[current_order]);
>>  fallback_mt = find_suitable_fallback(area, current_order,
>>  start_migratetype, false, _steal);
>> 

Re: [PATCH] mm: remove a redundant condition in the for loop

[Vlastimil Babka]

On 06/19/2017 03:54 PM, Hao Lee wrote:
> The variable current_order decreases from MAX_ORDER-1 to order, so the
> condition current_order <= MAX_ORDER-1 is always true.
> 
> Signed-off-by: Hao Lee 

Sounds right.

Acked-by: Vlastimil Babka 

> ---
>  mm/page_alloc.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2302f25..9120c2b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int 
> order, int start_migratetype)
>   bool can_steal;
>  
>   /* Find the largest possible block of pages in the other list */
> - for (current_order = MAX_ORDER-1;
> - current_order >= order && current_order <= 
> MAX_ORDER-1;
> - --current_order) {
> + for (current_order = MAX_ORDER-1; current_order >= order;
> + --current_order) {
>   area = &(zone->free_area[current_order]);
>   fallback_mt = find_suitable_fallback(area, current_order,
>   start_migratetype, false, _steal);
> 

Re: [PATCH] mm: remove a redundant condition in the for loop

[Vlastimil Babka]

On 06/19/2017 03:54 PM, Hao Lee wrote:
> The variable current_order decreases from MAX_ORDER-1 to order, so the
> condition current_order <= MAX_ORDER-1 is always true.
> 
> Signed-off-by: Hao Lee 

Sounds right.

Acked-by: Vlastimil Babka 

> ---
>  mm/page_alloc.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2302f25..9120c2b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int 
> order, int start_migratetype)
>   bool can_steal;
>  
>   /* Find the largest possible block of pages in the other list */
> - for (current_order = MAX_ORDER-1;
> - current_order >= order && current_order <= 
> MAX_ORDER-1;
> - --current_order) {
> + for (current_order = MAX_ORDER-1; current_order >= order;
> + --current_order) {
>   area = &(zone->free_area[current_order]);
>   fallback_mt = find_suitable_fallback(area, current_order,
>   start_migratetype, false, _steal);
>