Re: [PATCH] mm/vmalloc: a slight change of compare target in __insert_vmap_area()

[Michal Hocko]

On Wed 24-05-17 18:03:47, Wei Yang wrote:
> The vmap RB tree store the elements in order and no overlap between any of
> them. The comparison in __insert_vmap_area() is to decide which direction
> the search should follow and make sure the new vmap_area is not overlap
> with any other.
> 
> Current implementation fails to do the overlap check.
> 
> When first "if" is not true, it means
> 
>     va->va_start >= tmp_va->va_end
> 
> And with the truth
> 
>     xxx->va_end > xxx->va_start
> 
> The deduction is
> 
>     va->va_end > tmp_va->va_start
> 
> which is the condition in second "if".
> 
> This patch changes a little of the comparison in __insert_vmap_area() to
> make sure it forbids the overlapped vmap_area.

Why do we care about overlapping vmap areas at this level. This is an
internal function and all the sanity checks should have been done by
that time AFAIR. Could you describe the problem which you are trying to
fix/address?

> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> ---
>  mm/vmalloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 0b057628a7ba..8087451cb332 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -360,9 +360,9 @@ 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)
> +		if (va->va_end <= tmp_va->va_start)
>  			p = &(*p)->rb_left;
> -		else if (va->va_end > tmp_va->va_start)
> +		else if (va->va_start >= tmp_va->va_end)
>  			p = &(*p)->rb_right;
>  		else
>  			BUG();
> -- 
> 2.11.0
> 
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-- 
Michal Hocko
SUSE Labs