Re: [RFC PATCH 2/3] mm/memory-tiers: Use page counter to track toptier memory usage

[Tim Chen]

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