[RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area

[RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area

[zijun_hu]

From: zijun_hu <zijun_hu@htc.com>

the KVA allocator organizes vmap_areas allocated by rbtree. in order to
insert a new vmap_area @i_va into the rbtree, walk around the rbtree from
root and compare the vmap_area @t_va met on the rbtree against @i_va; walk
toward the left branch of @t_va if @i_va is lower than @t_va, and right
branch if higher, otherwise handle this error case since @i_va has overlay
with @t_va; however, __insert_vmap_area() don't follow the desired
procedure rightly, moreover, it includes a meaningless else if condition
and a redundant else branch as shown by comments in below code segments:
static void __insert_vmap_area(struct vmap_area *va)
{
as a internal interface parameter, we assume vmap_area @va has nonzero size
...
			if (va->va_start < tmp->va_end)
					p = &(*p)->rb_left;
			else if (va->va_end > tmp->va_start)
					p = &(*p)->rb_right;
this else if condition is always true and meaningless due to
va->va_end > va->va_start >= tmp_va->va_end > tmp_va->va_start normally
			else
					BUG();
this BUG() is meaningless too due to never be reached normally
...
}

it looks like the else if condition and else branch are canceled. no errors
are caused since the vmap_area @va to insert as a internal interface
parameter doesn't have overlay with any one on the rbtree normally. however
 __insert_vmap_area() looks weird and really has several logic errors as
pointed out above when it is viewed as a separate function.

fix by walking around vmap_area rbtree as described above to insert
a vmap_area.

BTW, (va->va_end == tmp_va->va_start) is consider as legal case since it
indicates vmap_area @va left neighbors with @tmp_va tightly.

Fixes: db64fe02258f ("mm: rewrite vmap layer")
Signed-off-by: zijun_hu <zijun_hu@htc.com>
---
 mm/vmalloc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 5daf3211b84f..8b80931654b7 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -321,10 +321,10 @@ static void __insert_vmap_area(struct vmap_area *va)
 
 		parent = *p;
 		tmp_va = rb_entry(parent, struct vmap_area, rb_node);
-		if (va->va_start < tmp_va->va_end)
-			p = &(*p)->rb_left;
-		else if (va->va_end > tmp_va->va_start)
-			p = &(*p)->rb_right;
+		if (va->va_end <= tmp_va->va_start)
+			p = &parent->rb_left;
+		else if (va->va_start >= tmp_va->va_end)
+			p = &parent->rb_right;
 		else
 			BUG();
 	}
-- 
1.9.1

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Re: [RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area

[Michal Hocko]

[Let's CC Nick who has written this code]

On Wed 12-10-16 22:30:13, zijun_hu wrote:
> From: zijun_hu <zijun_hu@htc.com>
> 
> the KVA allocator organizes vmap_areas allocated by rbtree. in order to
> insert a new vmap_area @i_va into the rbtree, walk around the rbtree from
> root and compare the vmap_area @t_va met on the rbtree against @i_va; walk
> toward the left branch of @t_va if @i_va is lower than @t_va, and right
> branch if higher, otherwise handle this error case since @i_va has overlay
> with @t_va; however, __insert_vmap_area() don't follow the desired
> procedure rightly, moreover, it includes a meaningless else if condition
> and a redundant else branch as shown by comments in below code segments:
> static void __insert_vmap_area(struct vmap_area *va)
> {
> as a internal interface parameter, we assume vmap_area @va has nonzero size
> ...
> 			if (va->va_start < tmp->va_end)
> 					p = &(*p)->rb_left;
> 			else if (va->va_end > tmp->va_start)
> 					p = &(*p)->rb_right;
> this else if condition is always true and meaningless due to
> va->va_end > va->va_start >= tmp_va->va_end > tmp_va->va_start normally
> 			else
> 					BUG();
> this BUG() is meaningless too due to never be reached normally
> ...
> }
> 
> it looks like the else if condition and else branch are canceled. no errors
> are caused since the vmap_area @va to insert as a internal interface
> parameter doesn't have overlay with any one on the rbtree normally. however
>  __insert_vmap_area() looks weird and really has several logic errors as
> pointed out above when it is viewed as a separate function.

I have tried to read this several times but I am completely lost to
understand what the actual bug is and how it causes vmap_area sorting to
misbehave. So is this a correctness issue, performance improvement or
theoretical fix for an incorrect input?

> fix by walking around vmap_area rbtree as described above to insert
> a vmap_area.
> 
> BTW, (va->va_end == tmp_va->va_start) is consider as legal case since it
> indicates vmap_area @va left neighbors with @tmp_va tightly.
> 
> Fixes: db64fe02258f ("mm: rewrite vmap layer")
> Signed-off-by: zijun_hu <zijun_hu@htc.com>
> ---
>  mm/vmalloc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 5daf3211b84f..8b80931654b7 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -321,10 +321,10 @@ static void __insert_vmap_area(struct vmap_area *va)
>  
>  		parent = *p;
>  		tmp_va = rb_entry(parent, struct vmap_area, rb_node);
> -		if (va->va_start < tmp_va->va_end)
> -			p = &(*p)->rb_left;
> -		else if (va->va_end > tmp_va->va_start)
> -			p = &(*p)->rb_right;
> +		if (va->va_end <= tmp_va->va_start)
> +			p = &parent->rb_left;
> +		else if (va->va_start >= tmp_va->va_end)
> +			p = &parent->rb_right;
>  		else
>  			BUG();
>  	}
> -- 
> 1.9.1

-- 
Michal Hocko
SUSE Labs

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Re: [RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area

[zijun_hu]

On 2016/10/12 22:46, Michal Hocko wrote:
> [Let's CC Nick who has written this code]
> 
> On Wed 12-10-16 22:30:13, zijun_hu wrote:
>> From: zijun_hu <zijun_hu@htc.com>
>>
>> the KVA allocator organizes vmap_areas allocated by rbtree. in order to
>> insert a new vmap_area @i_va into the rbtree, walk around the rbtree from
>> root and compare the vmap_area @t_va met on the rbtree against @i_va; walk
>> toward the left branch of @t_va if @i_va is lower than @t_va, and right
>> branch if higher, otherwise handle this error case since @i_va has overlay
>> with @t_va; however, __insert_vmap_area() don't follow the desired
>> procedure rightly, moreover, it includes a meaningless else if condition
>> and a redundant else branch as shown by comments in below code segments:
>> static void __insert_vmap_area(struct vmap_area *va)
>> {
>> as a internal interface parameter, we assume vmap_area @va has nonzero size
>> ...
>> 			if (va->va_start < tmp->va_end)
>> 					p = &(*p)->rb_left;
>> 			else if (va->va_end > tmp->va_start)
>> 					p = &(*p)->rb_right;
>> this else if condition is always true and meaningless due to
>> va->va_end > va->va_start >= tmp_va->va_end > tmp_va->va_start normally
>> 			else
>> 					BUG();
>> this BUG() is meaningless too due to never be reached normally
>> ...
>> }
>>
>> it looks like the else if condition and else branch are canceled. no errors
>> are caused since the vmap_area @va to insert as a internal interface
>> parameter doesn't have overlay with any one on the rbtree normally. however
>>  __insert_vmap_area() looks weird and really has several logic errors as
>> pointed out above when it is viewed as a separate function.
> 
> I have tried to read this several times but I am completely lost to
> understand what the actual bug is and how it causes vmap_area sorting to
> misbehave. So is this a correctness issue, performance improvement or
> theoretical fix for an incorrect input?
> 

there are several logic errors for this function in current code:

current code is :

static void __insert_vmap_area(struct vmap_area *va)
{
...

		if (va->va_start < tmp->va_end)
			p = &(*p)->rb_left;
		else if (va->va_end > tmp->va_start)
			p = &(*p)->rb_right;
		else
			BUG();
...
}

the current code is equivalent with the following code

static void __insert_vmap_area(struct vmap_area *va)
{
...
		if (va->va_start < tmp->va_end)
			p = &(*p)->rb_left;
		else
			p = &(*p)->rb_right;
...
}

as shown above, for current code :
this else if (va->va_end > tmp->va_start) is meaningless since it is always true
the else branch BUG(); is meaningless too since it never be reached
it seems there are logic error in the function

the code we expect should be as follows:

static void __insert_vmap_area(struct vmap_area *va)
{
...
		if (va->va_end <= tmp_va->va_start)
			p = &(*p)->rb_left;
		else if (va->va_start >= tmp_va->va_end)
			p = &(*p)->rb_right;
  		else
  			BUG();
...
}

>> fix by walking around vmap_area rbtree as described above to insert
>> a vmap_area.
>>
>> BTW, (va->va_end == tmp_va->va_start) is consider as legal case since it
>> indicates vmap_area @va left neighbors with @tmp_va tightly.
>>
>> Fixes: db64fe02258f ("mm: rewrite vmap layer")
>> Signed-off-by: zijun_hu <zijun_hu@htc.com>
>> ---
>>  mm/vmalloc.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 5daf3211b84f..8b80931654b7 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -321,10 +321,10 @@ static void __insert_vmap_area(struct vmap_area *va)
>>  
>>  		parent = *p;
>>  		tmp_va = rb_entry(parent, struct vmap_area, rb_node);
>> -		if (va->va_start < tmp_va->va_end)
>> -			p = &(*p)->rb_left;
>> -		else if (va->va_end > tmp_va->va_start)
>> -			p = &(*p)->rb_right;
>> +		if (va->va_end <= tmp_va->va_start)
>> +			p = &parent->rb_left;
>> +		else if (va->va_start >= tmp_va->va_end)
>> +			p = &parent->rb_right;
>>  		else
>>  			BUG();
>>  	}
>> -- 
>> 1.9.1
> 


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Re: [RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area

[zijun_hu]

Hi Nicholas,

i find __insert_vmap_area() is introduced by you
could you offer comments for this patch related to that funciton

thanks

On 10/12/2016 10:46 PM, Michal Hocko wrote:
> [Let's CC Nick who has written this code]
> 
> On Wed 12-10-16 22:30:13, zijun_hu wrote:
>> From: zijun_hu <zijun_hu@htc.com>
>>
>> the KVA allocator organizes vmap_areas allocated by rbtree. in order to
>> insert a new vmap_area @i_va into the rbtree, walk around the rbtree from
>> root and compare the vmap_area @t_va met on the rbtree against @i_va; walk
>> toward the left branch of @t_va if @i_va is lower than @t_va, and right
>> branch if higher, otherwise handle this error case since @i_va has overlay
>> with @t_va; however, __insert_vmap_area() don't follow the desired
>> procedure rightly, moreover, it includes a meaningless else if condition
>> and a redundant else branch as shown by comments in below code segments:
>> static void __insert_vmap_area(struct vmap_area *va)
>> {
>> as a internal interface parameter, we assume vmap_area @va has nonzero size
>> ...
>> 			if (va->va_start < tmp->va_end)
>> 					p = &(*p)->rb_left;
>> 			else if (va->va_end > tmp->va_start)
>> 					p = &(*p)->rb_right;
>> this else if condition is always true and meaningless due to
>> va->va_end > va->va_start >= tmp_va->va_end > tmp_va->va_start normally
>> 			else
>> 					BUG();
>> this BUG() is meaningless too due to never be reached normally
>> ...
>> }
>>
>> it looks like the else if condition and else branch are canceled. no errors
>> are caused since the vmap_area @va to insert as a internal interface
>> parameter doesn't have overlay with any one on the rbtree normally. however
>>  __insert_vmap_area() looks weird and really has several logic errors as
>> pointed out above when it is viewed as a separate function.
> 
> I have tried to read this several times but I am completely lost to
> understand what the actual bug is and how it causes vmap_area sorting to
> misbehave. So is this a correctness issue, performance improvement or
> theoretical fix for an incorrect input?
> 
>> fix by walking around vmap_area rbtree as described above to insert
>> a vmap_area.
>>
>> BTW, (va->va_end == tmp_va->va_start) is consider as legal case since it
>> indicates vmap_area @va left neighbors with @tmp_va tightly.
>>
>> Fixes: db64fe02258f ("mm: rewrite vmap layer")
>> Signed-off-by: zijun_hu <zijun_hu@htc.com>
>> ---
>>  mm/vmalloc.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 5daf3211b84f..8b80931654b7 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -321,10 +321,10 @@ static void __insert_vmap_area(struct vmap_area *va)
>>  
>>  		parent = *p;
>>  		tmp_va = rb_entry(parent, struct vmap_area, rb_node);
>> -		if (va->va_start < tmp_va->va_end)
>> -			p = &(*p)->rb_left;
>> -		else if (va->va_end > tmp_va->va_start)
>> -			p = &(*p)->rb_right;
>> +		if (va->va_end <= tmp_va->va_start)
>> +			p = &parent->rb_left;
>> +		else if (va->va_start >= tmp_va->va_end)
>> +			p = &parent->rb_right;
>>  		else
>>  			BUG();
>>  	}
>> -- 
>> 1.9.1
> 


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Re: [RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area

[zijun_hu]

On 10/13/2016 02:39 PM, zijun_hu wrote:

Hi Nicholas,
could you give some comments for this patch?

thanks a lot
> Hi Nicholas,
> 
> i find __insert_vmap_area() is introduced by you
> could you offer comments for this patch related to that funciton
> 
> thanks
> 
> On 10/12/2016 10:46 PM, Michal Hocko wrote:
>> [Let's CC Nick who has written this code]
>>
>> On Wed 12-10-16 22:30:13, zijun_hu wrote:
>>> From: zijun_hu <zijun_hu@htc.com>
>>>
>>> the KVA allocator organizes vmap_areas allocated by rbtree. in order to
>>> insert a new vmap_area @i_va into the rbtree, walk around the rbtree from
>>> root and compare the vmap_area @t_va met on the rbtree against @i_va; walk
>>> toward the left branch of @t_va if @i_va is lower than @t_va, and right
>>> branch if higher, otherwise handle this error case since @i_va has overlay
>>> with @t_va; however, __insert_vmap_area() don't follow the desired
>>> procedure rightly, moreover, it includes a meaningless else if condition
>>> and a redundant else branch as shown by comments in below code segments:
>>> static void __insert_vmap_area(struct vmap_area *va)
>>> {
>>> as a internal interface parameter, we assume vmap_area @va has nonzero size
>>> ...
>>> 			if (va->va_start < tmp->va_end)
>>> 					p = &(*p)->rb_left;
>>> 			else if (va->va_end > tmp->va_start)
>>> 					p = &(*p)->rb_right;
>>> this else if condition is always true and meaningless due to
>>> va->va_end > va->va_start >= tmp_va->va_end > tmp_va->va_start normally
>>> 			else
>>> 					BUG();
>>> this BUG() is meaningless too due to never be reached normally
>>> ...
>>> }
>>>
>>> it looks like the else if condition and else branch are canceled. no errors
>>> are caused since the vmap_area @va to insert as a internal interface
>>> parameter doesn't have overlay with any one on the rbtree normally. however
>>>  __insert_vmap_area() looks weird and really has several logic errors as
>>> pointed out above when it is viewed as a separate function.
>>
>> I have tried to read this several times but I am completely lost to
>> understand what the actual bug is and how it causes vmap_area sorting to
>> misbehave. So is this a correctness issue, performance improvement or
>> theoretical fix for an incorrect input?
>>
>>> fix by walking around vmap_area rbtree as described above to insert
>>> a vmap_area.
>>>
>>> BTW, (va->va_end == tmp_va->va_start) is consider as legal case since it
>>> indicates vmap_area @va left neighbors with @tmp_va tightly.
>>>
>>> Fixes: db64fe02258f ("mm: rewrite vmap layer")
>>> Signed-off-by: zijun_hu <zijun_hu@htc.com>
>>> ---
>>>  mm/vmalloc.c | 8 ++++----
>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index 5daf3211b84f..8b80931654b7 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -321,10 +321,10 @@ static void __insert_vmap_area(struct vmap_area *va)
>>>  
>>>  		parent = *p;
>>>  		tmp_va = rb_entry(parent, struct vmap_area, rb_node);
>>> -		if (va->va_start < tmp_va->va_end)
>>> -			p = &(*p)->rb_left;
>>> -		else if (va->va_end > tmp_va->va_start)
>>> -			p = &(*p)->rb_right;
>>> +		if (va->va_end <= tmp_va->va_start)
>>> +			p = &parent->rb_left;
>>> +		else if (va->va_start >= tmp_va->va_end)
>>> +			p = &parent->rb_right;
>>>  		else
>>>  			BUG();
>>>  	}
>>> -- 
>>> 1.9.1
>>
> 


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