[RFC PATCH v2 1/1] mm/vmalloc.c: correct a few logic error for __insert_vmap_area()

From: zijun_hu
Date: Tue Sep 27 2016 - 09:41:38 EST

From: zijun_hu <zijun_hu@xxxxxxx>

__insert_vmap_area() has a few obvious logic errors as shown by comments
within below code segments
static void __insert_vmap_area(struct vmap_area *va)
as a internal function 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
this BUG() is meaningless too due to never be touched normally

the function don't implement the below desire behavior based on context
if the vmap_area @va to be inserted is lower than that on the rbtree then
we walk around the left branch of the given rbtree node; else if higher
then right branch; else the former vmap_area has overlay with the latter
then the existing BUG() is triggered

it is fixed by correcting vmap_area rbtree walk manner as mentioned above
BTW, we consider (va->va_end == tmp_va->va_start) as legal case since it
indicate vmap_area @va neighbors with @tmp_va tightly

Fixes: db64fe02258f ("mm: rewrite vmap layer")
Signed-off-by: zijun_hu <zijun_hu@xxxxxxx>
Hi npiggin,
could you offer some comments for this patch since __insert_vmap_area()
was introduced by you?
thanks a lot

Changes in v2:
- more detailed commit message is provided

mm/vmalloc.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 91f44e78c516..cc6ecd60cc0e 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;