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@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?
>
>> 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
>
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