Re: [-mm][PATCH 8/10] fix shmem page migration incorrectness onmemcgroup

[Daisuke Nishimura]

On Fri, 27 Jun 2008 17:52:01 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Fri, 27 Jun 2008 16:57:56 +0900
> "MinChan Kim" <minchan.kim@xxxxxxxxx> wrote:
> 
> > On Fri, Jun 27, 2008 at 2:41 PM, KOSAKI Motohiro
> > <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> > >> > mem_cgroup_uncharge() against old page is done after radix-tree-replacement.
> > >> > And there were special handling to ingore swap-cache page. But, shmem can
> > >> > be swap-cache and file-cache at the same time. Chekcing PageSwapCache() is
> > >> > not correct here. Check PageAnon() instead.
> > >>
> > >> When/How shmem can be both swap-cache and file-cache ?
> > >> I can't understand that situation.
> > >
> > > Hi
> > >
> > > see,
> > >
> > > shmem_writepage()
> > >   -> add_to_swap_cache()
> > >      -> SetPageSwapCache()
> > >
> > >
> > > BTW: his file-cache mean !Anon, not mean !SwapBacked.
> > 
> > Hi KOSAKI-san.
> > Thanks for explaining.
> > 
> > In the migrate_page_move_mapping, the page was already locked in unmap_and_move.
> > Also, we have a lock for that page for calling shmem_writepage.
> > 
> > So I think race problem between shmem_writepage and
> > migrate_page_move_mapping don't occur.
> > But I am not sure I am right.
> > 
> > If I am wrong, could you tell me when race problem happen ? :)
> > 
> You are right. I misundestood the swap/shmem code. there is no race.
> Hmm...
> 
> But situation is a bit complicated.
> - shmem's page is charged as file-cache.
> - shmem's swap cache is still charged by mem_cgroup_cache_charge() because
>   it's implicitly (to memcg) converted to swap cache. 
> - anon's swap cache is charged by mem_cgroup_uncharge_cache_page()
> 
I'm sorry if I misunderstand something.

I think anon's swap cache is:

- charged by nowhere as "cache".
  If anon pages are also on swap cache, charges for them remain charged
  even when mem_cgroup_uncharge_page() is called, because it checks PG_swapcache.
  So, as a result, anon's swap cache is charged.
- uncharged by memcgroup_uncharge_page() in __delete_from_swap_cache()
  after clearing PG_swapcache.

right?

> So, uncharging swap-cache of shmem by mem_cgroup_uncharge_cache_page() is valid.
> Checking PageSwapCache() was bad and Cheking PageAnon() is good.
> (From maintainance view)
> 
agree.


Thanks,
Daisuke Nishimura.
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