linux-kernel - Re: [RFC PATCH 1/5] mm/vmalloc.c: correct a few logic error for __insert_vmap

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Message-ID: <57E33581.5030409@zoho.com>
Date:   Thu, 22 Sep 2016 09:36:01 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, zijun_hu@....com,
        tj@...nel.org, mingo@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, mgorman@...hsingularity.net
Subject: Re: [RFC PATCH 1/5] mm/vmalloc.c: correct a few logic error for
 __insert_vmap_area()

On 09/21/2016 12:23 PM, zijun_hu wrote:
> From: zijun_hu <zijun_hu@....com>
> 
> correct a few logic error for __insert_vmap_area() since the else
> if condition is always true and meaningless
> 
> in order to fix this issue, if vmap_area inserted is lower than one
> on rbtree then walk around left branch; if higher then right branch
> otherwise intersects with the other then BUG_ON() is triggered
> 
> Signed-off-by: zijun_hu <zijun_hu@....com>
i give more explanation to the intent of my change
any comments is welcome
> ---
>  mm/vmalloc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 91f44e7..cc6ecd6 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;
this else if condition is always true and meaningless as long as there are no
zero sized vamp_area due to the following expression
va->va_end > va->va_start >= tmp_va->va_end > tmp_va->va_start

> +		if (va->va_end <= tmp_va->va_start)
> +			p = &parent->rb_left;
if the vamp_area to be inserted is lower than that on the rbtree then
we walk around the left branch of the node given
consider va->va_end == tmp_va->va_start as legal case which represent
two neighbor areas tightly
BTW, the available range of a vmap area include the start boundary not the
end, namely, [start, end)
> +		else if (va->va_start >= tmp_va->va_end)
> +			p = &parent->rb_right;
if the vamp_area to be inserted is higher than that on the rbtree then
we walk around the right branch of the node given
>  		else
>  			BUG();
this indicate the vamp_area to be inserted have intersects with that on the rbtree
then we remain the BUG() logic
>  	}
>