Re: [RFC][BUGFIX][PATCH] memcg: fix underflow of mapped_file stat

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* nishimura@mxp.nes.nec.co.jp <nishimura@mxp.nes.nec.co.jp> [2010-04-13 13:42:07]:

> Hi.
> 
> When I was testing page migration, I found underflow problem of "mapped_file" field
> in memory.stat. This is a fix for the problem.
> 
> This patch is based on mmotm-2010-04-05-16-09, and IIUC it conflicts with Mel's
> compaction patches, so I send it as RFC for now. After next mmotm, which will
> include those patches, I'll update and resend this patch.
> 
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> page_add_file_rmap(), which can be called from remove_migration_ptes(), is
> assumed to increment memcg's stat of mapped file. But on success of page
> migration, the newpage(mapped file) has not been charged yet, so the stat will
> not be incremented. This behavior leads to underflow of memcg's stat because
> when the newpage is unmapped afterwards, page_remove_rmap() decrements the stat.
> This problem doesn't happen on failure path of page migration, because the old
> page(mapped file) hasn't been uncharge at the point of remove_migration_ptes().
> This patch fixes this problem by calling commit_charge(mem_cgroup_end_migration)
> before remove_migration_ptes().
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> ---
>  mm/migrate.c |   19 ++++++++++++++-----
>  1 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 5938db5..915c35e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -485,7 +485,8 @@ static int fallback_migrate_page(struct address_space *mapping,
>   *   < 0 - error code
>   *  == 0 - success
>   */
> -static int move_to_new_page(struct page *newpage, struct page *page)
> +static int move_to_new_page(struct page *newpage, struct page *page,
> +						struct mem_cgroup *mem)
>  {
>  	struct address_space *mapping;
>  	int rc;
> @@ -520,9 +521,16 @@ static int move_to_new_page(struct page *newpage, struct page *page)
>  	else
>  		rc = fallback_migrate_page(mapping, newpage, page);
> 
> -	if (!rc)
> +	if (!rc) {
> +		/*
> +		 * On success of page migration, the newpage has not been
> +		 * charged yet, so we must call end_migration() before
> +		 * remove_migration_ptes() to update stats of mapped file
> +		 * properly.
> +		 */
> +		mem_cgroup_end_migration(mem, page, newpage);
>  		remove_migration_ptes(page, newpage);
> -	else
> +	} else
>  		newpage->mapping = NULL;
> 
>  	unlock_page(newpage);
> @@ -633,7 +641,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> 
>  skip_unmap:
>  	if (!page_mapped(page))
> -		rc = move_to_new_page(newpage, page);
> +		rc = move_to_new_page(newpage, page, mem);

Why do we need to pass mem, won't try_get_mem_cgroup_from_page() help?
Is it cost versus space tradeoff.

> 
>  	if (rc)
>  		remove_migration_ptes(page, page);
> @@ -641,7 +649,8 @@ rcu_unlock:
>  	if (rcu_locked)
>  		rcu_read_unlock();
>  uncharge:
> -	if (!charge)
> +	if (rc)
> +		/* On success of page migration, we've alread called it */

Comment is not clear to me, but the code is :)

>  		mem_cgroup_end_migration(mem, page, newpage);
>  unlock:
>  	unlock_page(page);
> -- 
> 1.6.4
> 
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-- 
	Three Cheers,
	Balbir