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Subject: Re: [PATCH v2 01/12] mm: Assign id to every memcg-aware shrinker
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Wed, 18 Apr 2018 18:02:26 +0300
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On 18.04.2018 17:32, Tetsuo Handa wrote:
> Kirill Tkhai wrote:
>> On 18.04.2018 17:14, Tetsuo Handa wrote:
>>> Kirill Tkhai wrote:
>>>> The patch introduces shrinker::id number, which is used to enumerate
>>>> memcg-aware shrinkers. The number start from 0, and the code tries
>>>> to maintain it as small as possible.
>>>>
>>>> This will be used as to represent a memcg-aware shrinkers in memcg
>>>> shrinkers map.
>>>
>>> I'm not reading this thread. But is there reason "id" needs to be managed
>>> using smallest numbers? Can't we use address of shrinker object as "id"
>>> (which will be sparse bitmap, and would be managed using linked list for now)?
>>
>> Yes, it's needed to have the smallest numbers, as next patches introduce
>> per-memcg bitmap containing ids of shrinkers.
> 
> If you use sparse bitmap (xbitmap ?), I think you can do it.

There is no implementation in kernel, and search gave me this link:
https://patchwork.kernel.org/patch/10128397/

The problem is that it may allocate memory, and hence to fail.
While adding an element to shrinker lists (and setting a bit
in bitmap) mustn't fail. So, it's not possible to use sparse bitmap.

Kirill