linux-kernel - Re: [PATCH] mark_page_accessed() in do_swap

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Date:	Thu, 15 Jan 2009 12:07:40 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	balbir@...ux.vnet.ibm.com,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	lizf@...fujitsu.com, menage@...gle.com
Subject: Re: [PATCH] mark_page_accessed() in do_swap_page() move latter than
 memcg charge

On Thu, 15 Jan 2009, KOSAKI Motohiro wrote:
> 
> sorry for late responce.
> 
> > > In this case we've hit a case where the page is valid and the pc is
> > > not. This does fix the problem, but won't this impact us getting
> > > correct reclaim stats and thus indirectly impact the working of
> > > pressure?
> > > 
> >  - If retruns NULL, only global LRU's status is updated. 
> > 
> > Because this page is not belongs to any memcg, we cannot update
> > any counters. But yes, your point is a concern.
> > 
> > Maybe moving acitvate_page() to
> > ==
> > do_swap_page()
> > {
> >     
> > - activate_page()
> >    mem_cgroup_try_charge()..
> >    ....
> >    mem_cgroup_commit_charge()....
> >    ....
> > +  activate_page()   
> > }
> > ==
> > is necessary. How do you think, kosaki ?
> 
> 
> OK. it makes sense. and my test found no bug.
> 
> ==
> 
> mark_page_accessed() update reclaim_stat statics.
> but currently, memcg charge is called after mark_page_accessed().
> 
> then, mark_page_accessed() don't update memcg statics correctly.

Statics?  "Stats" is a good abbreviation for statistics,
but statics are something else.

> 
> fixing here.
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Cc: Balbir Singh <balbir@...ux.vnet.ibm.com>
> 
> ---
>  mm/memory.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Index: b/mm/memory.c
> ===================================================================
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2426,8 +2426,6 @@ static int do_swap_page(struct mm_struct
>  		count_vm_event(PGMAJFAULT);
>  	}
>  
> -	mark_page_accessed(page);
> -
>  	lock_page(page);
>  	delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
>  
> @@ -2480,6 +2478,8 @@ static int do_swap_page(struct mm_struct
>  		try_to_free_swap(page);
>  	unlock_page(page);
>  
> +	mark_page_accessed(page);
> +
>  	if (write_access) {
>  		ret |= do_wp_page(mm, vma, address, page_table, pmd, ptl, pte);
>  		if (ret & VM_FAULT_ERROR)

This catches my eye, because I'd discussed with Nick and was going to
send in a patch which entirely _removes_ this mark_page_accessed call
from do_swap_page (and replaces follow_page's mark_page_accessed call
by a pte_mkyoung): they seem inconsistent to me, in the light of
bf3f3bc5e734706730c12a323f9b2068052aa1f0 mm: don't mark_page_accessed
in fault path.

Though I need to give it another think through first: the situation
is muddied by the way we (rightly) don't bother to do the mark_page_
accessed on Anon in zap_pte_range anyway; and anon/swap has an
independent lifecycle now with the separate swapbacked LRUs.

What do you think?  I didn't look further into your memcg situation,
and what this patch is about: I'm unclear whether my patch to remove
that mark_page_accessed would solve your problem, or mess you up.

Hugh
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