linux-kernel - Re: [BUGFIX][PATCH v3] memcg: fix memory migration of shmem swapcache

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Message-ID: <20110106054200.GG3722@balbir.in.ibm.com>
Date:	Thu, 6 Jan 2011 11:12:00 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [BUGFIX][PATCH v3] memcg: fix memory migration of shmem
 swapcache

* nishimura@....nes.nec.co.jp <nishimura@....nes.nec.co.jp> [2011-01-06 12:34:15]:

> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index 159a076..cc5a8fd 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -93,7 +93,7 @@ extern int
> > >  mem_cgroup_prepare_migration(struct page *page,
> > >        struct page *newpage, struct mem_cgroup **ptr);
> > >  extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
> > > -       struct page *oldpage, struct page *newpage);
> > > +       struct page *oldpage, struct page *newpage, bool success);
> > 
> > The term "success" implies present or future tense.
> > The event this variable cares about in the past so "succeed" might be
> > a more appropriate term.
> > Sorry to be picky about the English but there is an important
> > distinction here since we don't have any comment about the variable.
> > 
> > Am I being too fussy?
> Not at all. Your comments are very helpful to make the code more readable :)
> 
> ===
> From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> 
> In current implimentation, mem_cgroup_end_migration() decides whether the page
> migration has succeeded or not by checking "oldpage->mapping".
> 
> But if we are tring to migrate a shmem swapcache, the page->mapping of it is
> NULL from the begining, so the check would be invalid.
> As a result, mem_cgroup_end_migration() assumes the migration has succeeded
> even if it's not, so "newpage" would be freed while it's not uncharged.
> 
> This patch fixes it by passing mem_cgroup_end_migration() the result of the
> page migration.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Reviewed-by: Minchan Kim <minchan.kim@...il.com>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> v2->v3
>   - s/success/succeed
> 
> v1->v2
>   - pass mem_cgroup_end_migration() "bool" instead of "int".
> 
>  include/linux/memcontrol.h |    5 ++---
>  mm/memcontrol.c            |    5 ++---
>  mm/migrate.c               |    2 +-
>  3 files changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 159a076..9f52b57 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -93,7 +93,7 @@ extern int
>  mem_cgroup_prepare_migration(struct page *page,
>  	struct page *newpage, struct mem_cgroup **ptr);
>  extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
> -	struct page *oldpage, struct page *newpage);
> +	struct page *oldpage, struct page *newpage, bool succeed);

Sorry for nit-picking but succeed is not as good as succeeded,
successful, successful_migration or migration_ok

> 
>  /*
>   * For memory reclaim.
> @@ -231,8 +231,7 @@ mem_cgroup_prepare_migration(struct page *page, struct page *newpage,
>  }
> 
>  static inline void mem_cgroup_end_migration(struct mem_cgroup *mem,
> -					struct page *oldpage,
> -					struct page *newpage)
> +		struct page *oldpage, struct page *newpage, bool succeed)
>  {
>  }
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 61678be..71a39bc 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2856,7 +2856,7 @@ int mem_cgroup_prepare_migration(struct page *page,
> 
>  /* remove redundant charge if migration failed*/
>  void mem_cgroup_end_migration(struct mem_cgroup *mem,
> -	struct page *oldpage, struct page *newpage)
> +	struct page *oldpage, struct page *newpage, bool succeed)
>  {
>  	struct page *used, *unused;
>  	struct page_cgroup *pc;
> @@ -2865,8 +2865,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *mem,
>  		return;
>  	/* blocks rmdir() */
>  	cgroup_exclude_rmdir(&mem->css);
> -	/* at migration success, oldpage->mapping is NULL. */
> -	if (oldpage->mapping) {
> +	if (!succeed) {
>  		used = oldpage;
>  		unused = newpage;
>  	} else {
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6ae8a66..be66b23 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -756,7 +756,7 @@ rcu_unlock:
>  		rcu_read_unlock();
>  uncharge:
>  	if (!charge)
> -		mem_cgroup_end_migration(mem, page, newpage);
> +		mem_cgroup_end_migration(mem, page, newpage, rc == 0);
>  unlock:
>  	unlock_page(page);
> 
> -- 
> 1.7.1
> 
> --
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-- 
	Three Cheers,
	Balbir
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