Re: [PATCH] vmscan: memcg needs may_swap (Re: [patch] vmscan: rename sc.may_swap to may_unmap)

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> memory cgroup has 2 calls to this shrink_zone.
>  1. memory usage hits the limit.
>  2. mem+swap usage hits the limit.
> 
> At "2", swap-out doesn't decrease the usage of mem+swap, then set may_swap=0.
> So, we want to kick out only file caches.
> But, we can reclaim file cache and "unmap file cache and reclaim it!" is 
> necessary even if may_swap=0.
> 
> Then, scanning only FILE LRU makes sense at may_swap=0 *if* memcg is
> the only user of may_swap=0.
> 
> Let's see others.
> 
>  - __zone_reclaim sets may_unmap to be 0 when they don't want swap-out.
>    .....can be replaced with may_swap.
> 
>  - shrink_all_memory sets may_swap to be 0. Is this called by hibernation ?
>    If you don't want to unmap file caches while hibernation, adding may_unmap
>    as *new* paramter makes sense, I think.
> 
> The change you proposed is for dropping unused SwapCache pages. Right ?
> But this will be dropped by kswapd if necessary.
> 
> As far as memcg concerns, scanning ANON LRU even when may_swap=0 is just
> a waste of cpu time.

this sentence just explain my intention.

1. memcg, zone_reclaim scanning ANON LRU is just waste of cpu.
2. kswapd and normal direct reclaim can reclaim stealed swapcache anyway.
   then above trick don't cause any system hang-up and performance degression.



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