Re: [RFC][BUGFIX][PATCH 2/2] memcg: fix file mapped underflow at migration (v3)

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 19 Apr 2010 17:07:01 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> Thank you for explaining in detail.
> 
> On Mon, 19 Apr 2010 13:18:17 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Mon, 19 Apr 2010 12:42:25 +0900
> > Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> > 
> > > Hmm, before going further, will you explain why we need a new PCG_MIGRATION flag ?
> > > What's the problem of v2 ?
> > > 
> > 
> > v2 can't handle migration-failure case of freed swapcache and the used page
> > was swapped-out case. I think.
> > 
> > All "page" in following is ANON.
> > 
> > 
> >      mem_cgroup_prepare_migration()
> > 	     charge against new page.
> >      
> >      try_to_unmap()
> >         -> mapcount goes down to 0.
> >              -> an old page is unchaged
> >      
> But old page isn't uncharged iff PageSwapCache, is it ?
> 
yes.


> >      move_to_new_page()
> >         -> may fail. (in some case.)   ----(*1)
> > 
> >      remap the old page to pte.
> > 
> >      mem_cgroup_end_migration()
> > 		(at success *1)
> > 		check charge for newpage is valid or not (*2)
> > 
> > 		(at fail *1)
> > 		uncharge new page.
> > 		What we should do for an old page. ---(*3)
> > 
> > At (*2). (*3), there are several cases.
> > 
> > (*2) migration was succeeded.
> >     1. The new page was successfully remapped.
> > 	-> Nothing to do.
> >     2. The new page was remapped but finally unmapped before (*3)
> > 	-> page_remove_rmap() will catch the event.
> >     3. The new page was not remapped.
> > 	-> page_remove_rmap() can't catch the event. end_migraion() has to
> > 	uncharge it.
> > 
> > (*3) migration was failed.
> >     1. The old page was successfully remapped.
> > 	-> We have to recharge against the old page. (But it may hit OOM.)
> >     2. The old page wasn't remapped.
> >         -> mapcount is 0. No new charge will happen.
> >     3. The old page wasn't remapped but SwapCache.
> >         -> mapcount is 0. We have to recharge against the old page (But it may hit OOM)
> > 
> hmm, we've done try_charge at this point, so why can we cause oom here ?
> 

v2 doesn't charge. That was the bug.
"may hit OOM" is an explanation for why current implementation is used.
(current implemnation == delayed commmit charge.)


> > Maybe other seqence I couldn't write will exist......IMHO, "we have to recharge it because
> > it's uncharged.." is bad idea. It seems hard to maintainace..
> > 
> > 
> > When we use MIGRATION flag.
> > After migaration.
> > 
> >     1. Agaisnt new page, we remove MIGRATION flag and try to uncharge() it again.
> > 
> >     2. Agaisnt old page, we remove MIGRATION flag and try to uncharge it again.
> > 
> > NOTE:  I noticed my v3 patch is buggy when the page-is-swapped-out case. It seems
> >        mem_cgroup_uncharge_swapcache() has to wait for migration ends or some
> >        other case handling. (Anyway, this race exists only after unlock_page(newpage).
> >        So, wait for MIGRATION ends in spin will not be very bad.)
> > 
> > 
> > To me, things are much simpler than now, we have to know what kind of magics behind us...
> > 
> > Maybe I can think of other tricks for handling them...but using a FLAG and prevent uncharge
> > is the simplest, I think.
> > 
> Anyway, I agree that current implementation is complicated and there might be
> some cases we are missing. MIGRATION flag can make it simpler.
> 
I think so.

> I have one concern for now. Reading the patch, the flag have influence on
> only anonymous pages, so we'd better to note it and I feel it strange to
> set(and clear) the flag of "old page" always(iow, even when !PageAnon)
> in prepare_migration.
> 

Hmm...Checking "Only Anon" is simpler ? It will have no meanings for migrating
file caches, but it may have some meanings for easy debugging. 
I think "mark it always but it's used only for anonymous page" is reasonable
(if it causes no bug.)


Thanks,
-Kame