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@xxxxxxx>
>>
>> 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@xxxxxxx>
>> ---
>>  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
>