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Subject: Re: [RFC PATCH 2/3] mm/memory-tiers: Use page counter to track
 toptier memory usage
From:   Tim Chen <tim.c.chen@linux.intel.com>
To:     Wei Xu <weixugc@google.com>
Cc:     Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Huang Ying <ying.huang@intel.com>,
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        Dan Williams <dan.j.williams@intel.com>,
        Feng Tang <feng.tang@intel.com>,
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        David Rientjes <rientjes@google.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Shakeel Butt <shakeelb@google.com>
Date:   Wed, 15 Jun 2022 21:12:47 -0700
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On Tue, 2022-06-14 at 17:30 -0700, Wei Xu wrote:


Thanks for your comments.

> When we don't know which pages are being charged, we should still
> charge the usage to toptier (assuming that toptier always include the
> default tier), e.g. from try_charge_memcg().
> 

I delayed the charging of the toptier a bit till we know which page
is being used and the memcg is being assigned to the page.  
That's when mem_cgroup_charge_toptier is invoked. 

Otherwise if we charge to toptier first, we will have additional 
work to deduct the count when pages used are not toptier.

> The idea is that when lower tier memory is not used, memcg->toptier
> and memcg->memory should have the same value. Otherwise, it can cause
> confusions about where the pages of (memcg->memory - memcg->toptier)
> go.

Any difference should be very small as the charge will go into toptier
too quickly. The values will be different even if
memcg->memory is read at slightly different time anyway.

Tim