linux-kernel - Re: [RFC PATCH V1 0/3] mm/damon: Add CMA minotor support

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Message-ID: <5c46548a-04b9-607e-f43a-63545131d336@redhat.com>
Date:   Fri, 18 Mar 2022 09:29:20 +0100
From:   David Hildenbrand <david@...hat.com>
To:     xhao@...ux.alibaba.com, sj@...nel.org
Cc:     rongwei.wang@...ux.alibaba.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [RFC PATCH V1 0/3] mm/damon: Add CMA minotor support

On 18.03.22 06:13, xhao@...ux.alibaba.com wrote:
> 
> On 3/18/22 12:42 AM, David Hildenbrand wrote:
>> On 17.03.22 08:03, Xin Hao wrote:
>>> Hi David,
>>>
>>> On 3/16/22 11:09 PM, David Hildenbrand wrote:
>>>> On 15.03.22 17:37, Xin Hao wrote:
>>>>
>>>> s/minotor/monitor/
>>> Thanks,  i will fix it.
>>>>> The purpose of these patches is to add CMA memory monitoring function.
>>>>> In some memory tight scenarios, it will be a good choice to release more
>>>>> memory by monitoring the CMA memory.
>>>> I'm sorry, but it's hard to figure out what the target use case should
>>>> be. Who will release CMA memory and how? Who will monitor that? What are
>>>> the "some memory tight scenarios"? What's the overall design goal?
>>> I may not be describing exactly what  i mean，My intention is to find out
>>> how much of the reserved CMA space is actually used and which is unused,
>>> For those that are not used, I understand that they can be released by
>>> cma_release(). Of course, This is just a little personal thought that I
>>> think is helpful for saving memory.
>> Hm, not quite. We can place movable allocations on cma areas, to be
>> migrated away once required for allocations via CMA. So just looking at
>> the pages allocated within a CMA area doesn't really tell you what's
>> actually going on.
> 
> I don't think so,  the damon not looking at the pages allocate, It is 
> constantly monitoring who is using CMA area pages through tracking page 
> access bit
> 
> in the kernel via the kdamond.x thread, So through damon, it can tell us 
> about  the hot and cold distribution of CMA memory.

I'm not sure I follow. With random movable pages being placed on the CMA
area, the mentioned use case of "cma_release()" to release pages doesn't
make sense to me.

I assume I'm missing the big picture -- and that should be properly
documented in the patch description. We don't add stuff just because it
could be used somehow, there should be a clear motivation how it can
actually be used.

-- 
Thanks,

David / dhildenb