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From:   sj@kernel.org
To:     David Hildenbrand <david@redhat.com>
Cc:     xhao@linux.alibaba.com, sj@kernel.org,
        rongwei.wang@linux.alibaba.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Baolin Wang <baolin.wang@linux.alibaba.com>
Subject: Re: [RFC PATCH V1 0/3] mm/damon: Add CMA minotor support
Date:   Fri, 18 Mar 2022 08:40:41 +0000
Message-Id: <20220318084041.25949-1-sj@kernel.org>
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On Fri, 18 Mar 2022 09:29:20 +0100 David Hildenbrand <david@redhat.com> wrote:

> On 18.03.22 06:13, xhao@linux.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.

Same opinion from my side.  The purpose and usage of this patch is unclear to
me.  Could you please clarify more, Xin?


Thanks,
SJ

> 
> -- 
> Thanks,
> 
> David / dhildenb