Re: [PATCH] mm: memcontrol: Let mem_cgroup_move_account() have effect only if MMU enabled

[Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>]

On Tue 03-03-15 06:42:01, Chen Gang wrote:
> When !MMU, it will report warning. The related warning with allmodconfig
> under c6x:

Does it even make any sense to enable CONFIG_MEMCG when !CONFIG_MMU?
Is anybody using this configuration and is it actually usable? My
knowledge about CONFIG_MMU is close to zero so I might be missing
something but I do not see a point into fixing compile warnings when
the whole subsystem is not usable in the first place.

> 
>     CC      mm/memcontrol.o
>   mm/memcontrol.c:2802:12: warning: 'mem_cgroup_move_account' defined but not used [-Wunused-function]
>    static int mem_cgroup_move_account(struct page *page,
>               ^
> 
> Signed-off-by: Chen Gang <gang.chen.5i5j-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> ---
>  mm/memcontrol.c | 172 ++++++++++++++++++++++++++++----------------------------
>  1 file changed, 86 insertions(+), 86 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0c86945..80f26f5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2785,92 +2785,6 @@ void mem_cgroup_split_huge_fixup(struct page *head)
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> -/**
> - * mem_cgroup_move_account - move account of the page
> - * @page: the page
> - * @nr_pages: number of regular pages (>1 for huge pages)
> - * @from: mem_cgroup which the page is moved from.
> - * @to:	mem_cgroup which the page is moved to. @from != @to.
> - *
> - * The caller must confirm following.
> - * - page is not on LRU (isolate_page() is useful.)
> - * - compound_lock is held when nr_pages > 1
> - *
> - * This function doesn't do "charge" to new cgroup and doesn't do "uncharge"
> - * from old cgroup.
> - */
> -static int mem_cgroup_move_account(struct page *page,
> -				   unsigned int nr_pages,
> -				   struct mem_cgroup *from,
> -				   struct mem_cgroup *to)
> -{
> -	unsigned long flags;
> -	int ret;
> -
> -	VM_BUG_ON(from == to);
> -	VM_BUG_ON_PAGE(PageLRU(page), page);
> -	/*
> -	 * The page is isolated from LRU. So, collapse function
> -	 * will not handle this page. But page splitting can happen.
> -	 * Do this check under compound_page_lock(). The caller should
> -	 * hold it.
> -	 */
> -	ret = -EBUSY;
> -	if (nr_pages > 1 && !PageTransHuge(page))
> -		goto out;
> -
> -	/*
> -	 * Prevent mem_cgroup_migrate() from looking at page->mem_cgroup
> -	 * of its source page while we change it: page migration takes
> -	 * both pages off the LRU, but page cache replacement doesn't.
> -	 */
> -	if (!trylock_page(page))
> -		goto out;
> -
> -	ret = -EINVAL;
> -	if (page->mem_cgroup != from)
> -		goto out_unlock;
> -
> -	spin_lock_irqsave(&from->move_lock, flags);
> -
> -	if (!PageAnon(page) && page_mapped(page)) {
> -		__this_cpu_sub(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED],
> -			       nr_pages);
> -		__this_cpu_add(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED],
> -			       nr_pages);
> -	}
> -
> -	if (PageWriteback(page)) {
> -		__this_cpu_sub(from->stat->count[MEM_CGROUP_STAT_WRITEBACK],
> -			       nr_pages);
> -		__this_cpu_add(to->stat->count[MEM_CGROUP_STAT_WRITEBACK],
> -			       nr_pages);
> -	}
> -
> -	/*
> -	 * It is safe to change page->mem_cgroup here because the page
> -	 * is referenced, charged, and isolated - we can't race with
> -	 * uncharging, charging, migration, or LRU putback.
> -	 */
> -
> -	/* caller should have done css_get */
> -	page->mem_cgroup = to;
> -	spin_unlock_irqrestore(&from->move_lock, flags);
> -
> -	ret = 0;
> -
> -	local_irq_disable();
> -	mem_cgroup_charge_statistics(to, page, nr_pages);
> -	memcg_check_events(to, page);
> -	mem_cgroup_charge_statistics(from, page, -nr_pages);
> -	memcg_check_events(from, page);
> -	local_irq_enable();
> -out_unlock:
> -	unlock_page(page);
> -out:
> -	return ret;
> -}
> -
>  #ifdef CONFIG_MEMCG_SWAP
>  static void mem_cgroup_swap_statistics(struct mem_cgroup *memcg,
>  					 bool charge)
> @@ -4822,6 +4736,92 @@ static struct page *mc_handle_file_pte(struct vm_area_struct *vma,
>  	return page;
>  }
>  
> +/**
> + * mem_cgroup_move_account - move account of the page
> + * @page: the page
> + * @nr_pages: number of regular pages (>1 for huge pages)
> + * @from: mem_cgroup which the page is moved from.
> + * @to:	mem_cgroup which the page is moved to. @from != @to.
> + *
> + * The caller must confirm following.
> + * - page is not on LRU (isolate_page() is useful.)
> + * - compound_lock is held when nr_pages > 1
> + *
> + * This function doesn't do "charge" to new cgroup and doesn't do "uncharge"
> + * from old cgroup.
> + */
> +static int mem_cgroup_move_account(struct page *page,
> +				   unsigned int nr_pages,
> +				   struct mem_cgroup *from,
> +				   struct mem_cgroup *to)
> +{
> +	unsigned long flags;
> +	int ret;
> +
> +	VM_BUG_ON(from == to);
> +	VM_BUG_ON_PAGE(PageLRU(page), page);
> +	/*
> +	 * The page is isolated from LRU. So, collapse function
> +	 * will not handle this page. But page splitting can happen.
> +	 * Do this check under compound_page_lock(). The caller should
> +	 * hold it.
> +	 */
> +	ret = -EBUSY;
> +	if (nr_pages > 1 && !PageTransHuge(page))
> +		goto out;
> +
> +	/*
> +	 * Prevent mem_cgroup_migrate() from looking at page->mem_cgroup
> +	 * of its source page while we change it: page migration takes
> +	 * both pages off the LRU, but page cache replacement doesn't.
> +	 */
> +	if (!trylock_page(page))
> +		goto out;
> +
> +	ret = -EINVAL;
> +	if (page->mem_cgroup != from)
> +		goto out_unlock;
> +
> +	spin_lock_irqsave(&from->move_lock, flags);
> +
> +	if (!PageAnon(page) && page_mapped(page)) {
> +		__this_cpu_sub(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED],
> +			       nr_pages);
> +		__this_cpu_add(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED],
> +			       nr_pages);
> +	}
> +
> +	if (PageWriteback(page)) {
> +		__this_cpu_sub(from->stat->count[MEM_CGROUP_STAT_WRITEBACK],
> +			       nr_pages);
> +		__this_cpu_add(to->stat->count[MEM_CGROUP_STAT_WRITEBACK],
> +			       nr_pages);
> +	}
> +
> +	/*
> +	 * It is safe to change page->mem_cgroup here because the page
> +	 * is referenced, charged, and isolated - we can't race with
> +	 * uncharging, charging, migration, or LRU putback.
> +	 */
> +
> +	/* caller should have done css_get */
> +	page->mem_cgroup = to;
> +	spin_unlock_irqrestore(&from->move_lock, flags);
> +
> +	ret = 0;
> +
> +	local_irq_disable();
> +	mem_cgroup_charge_statistics(to, page, nr_pages);
> +	memcg_check_events(to, page);
> +	mem_cgroup_charge_statistics(from, page, -nr_pages);
> +	memcg_check_events(from, page);
> +	local_irq_enable();
> +out_unlock:
> +	unlock_page(page);
> +out:
> +	return ret;
> +}
> +
>  static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
>  		unsigned long addr, pte_t ptent, union mc_target *target)
>  {
> -- 
> 1.9.3
> 
> 

-- 
Michal Hocko
SUSE Labs