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

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Date:   Thu, 22 Sep 2016 07:10:07 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     zijun_hu@....com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, tj@...nel.org,
        mingo@...nel.org, iamjoonsoo.kim@....com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH 1/5] mm/vmalloc.c: correct a few logic error for
 __insert_vmap_area()

On 2016/9/22 6:45, David Rientjes wrote:
> On Thu, 22 Sep 2016, zijun_hu wrote:
> 
>>>> 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
>>>>
>>>
>>> Under normal operation, you're right that the "else if" conditional should 
>>> always succeed: we don't want to BUG() unless there's a bug.  The original 
>>> code can catch instances when va->va_start == tmp_va->va_end where we 
>>> should BUG().  Your code silently ignores it.
>>>
>> Hmm, the BUG_ON() appears in the original code, i don't introduce it.
>> it maybe be better to consider va->va_start == tmp_va->va_end as normal case
>> and should not BUG_ON() it since the available range of vmap_erea include
>> the start boundary but the end, BTW, represented as [start, end)
>>
> 
> We don't support inserting when va->va_start == tmp_va->va_end, plain and 
> simple.  There's no reason to do so.  NACK to the patch.
> 
i am sorry i disagree with you because
1) in almost all context of vmalloc, original logic treat the special case as normal
   for example, __find_vmap_area() or alloc_vmap_area()
2) don't use the limited vmap area effectively, it maybe causes BUG_ON() easy
3) consider below case
   it provided there have been two vmap_areas [4, 12) and [20, 28), what will happens
   when alloc_vmap_area(8, 4, 6, 24,...)?  should we use [12,20) for our request?