linux-kernel - Re: [PATCH] mm: initialize variable for mem_cgroup_end_page

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Date:	Thu, 30 Oct 2014 09:27:12 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	riel@...hat.com, hannes@...xchg.org, peterz@...radead.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: initialize variable for mem_cgroup_end_page_stat

On Wed 29-10-14 21:44:24, Sasha Levin wrote:
> Commit "mm: memcontrol: fix missed end-writeback page accounting" has changed
> the behaviour of mem_cgroup_begin_page_stat() to not always set the "locked"
> parameter.
> 
> We should initialize it at the callers to prevent garbage being used in a
> later call to mem_cgroup_end_page_stat().

The contract is that if the returned memcg is non-NULL then the locked
is always initialized. Nobody but mem_cgroup_end_page_stat should touch
this variable and this function makes sure it uses it properly. Similar
applies to flags which is initialized only if we really take the slow
path (has a meaning only if locked == true).

So this is not really needed. Was this triggered by a compiler warning?

> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  mm/page-writeback.c |    4 ++--
>  mm/rmap.c           |    4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 19ceae8..7a02c97 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2329,7 +2329,7 @@ int test_clear_page_writeback(struct page *page)
>  	struct address_space *mapping = page_mapping(page);
>  	unsigned long memcg_flags;
>  	struct mem_cgroup *memcg;
> -	bool locked;
> +	bool locked = false;
>  	int ret;
>  
>  	memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);
> @@ -2366,7 +2366,7 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
>  	struct address_space *mapping = page_mapping(page);
>  	unsigned long memcg_flags;
>  	struct mem_cgroup *memcg;
> -	bool locked;
> +	bool locked = false;
>  	int ret;
>  
>  	memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 19886fb..4a4dc84 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1044,7 +1044,7 @@ void page_add_file_rmap(struct page *page)
>  {
>  	struct mem_cgroup *memcg;
>  	unsigned long flags;
> -	bool locked;
> +	bool locked = false;
>  
>  	memcg = mem_cgroup_begin_page_stat(page, &locked, &flags);
>  	if (atomic_inc_and_test(&page->_mapcount)) {
> @@ -1058,7 +1058,7 @@ static void page_remove_file_rmap(struct page *page)
>  {
>  	struct mem_cgroup *memcg;
>  	unsigned long flags;
> -	bool locked;
> +	bool locked = false;
>  
>  	memcg = mem_cgroup_begin_page_stat(page, &locked, &flags);
>  
> -- 
> 1.7.10.4
> 

-- 
Michal Hocko
SUSE Labs
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