[PATCH][RFC] mm: memcontrol: fix a unbalance uncharged count

[PATCH][RFC] mm: memcontrol: fix a unbalance uncharged count

[roy.qing.li]

From: Li RongQing <roy.qing.li@gmail.com>

I see the number of page of hpage_nr_pages(page) is charged if page is
transparent huge or hugetlbfs pages; but when uncharge a huge page,
(1<<compound_order) page is uncharged, and maybe hpage_nr_pages(page) is
not same as 1<<compound_order.

And remove VM_BUG_ON_PAGE(!PageTransHuge(page), page); since
PageTransHuge(page) always is true, when this VM_BUG_ON_PAGE is called.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
---
 mm/memcontrol.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 12aaadd..28c0137 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5453,8 +5453,7 @@ static void uncharge_list(struct list_head *page_list)
 		}
 
 		if (PageTransHuge(page)) {
-			nr_pages <<= compound_order(page);
-			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
+			nr_pages = hpage_nr_pages(page);
 			nr_huge += nr_pages;
 		}
 
-- 
2.1.4

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Re: [PATCH][RFC] mm: memcontrol: fix a unbalance uncharged count

[Michal Hocko]

On Tue 31-05-16 17:38:32, roy.qing.li@gmail.com wrote:
> From: Li RongQing <roy.qing.li@gmail.com>
> 
> I see the number of page of hpage_nr_pages(page) is charged if page is
> transparent huge or hugetlbfs pages; but when uncharge a huge page,

hugetlb pages do not get charged to the memcg. They have their own
hugetlbfscg controller.

> (1<<compound_order) page is uncharged, and maybe hpage_nr_pages(page) is
> not same as 1<<compound_order.

This should never happen. So this is not a fix. I guess it would a clean
up though.

> And remove VM_BUG_ON_PAGE(!PageTransHuge(page), page); since
> PageTransHuge(page) always is true, when this VM_BUG_ON_PAGE is called.

I guess we can drop this BUG_ON because nobody should be be seeing the
page at this stage so we cannot race with a THP split up. But I would
really have to look closer. This would be a patch on its own with the
full explanation though, IMHO.

> Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
> ---
>  mm/memcontrol.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 12aaadd..28c0137 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5453,8 +5453,7 @@ static void uncharge_list(struct list_head *page_list)
>  		}
>  
>  		if (PageTransHuge(page)) {
> -			nr_pages <<= compound_order(page);
> -			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> +			nr_pages = hpage_nr_pages(page);
>  			nr_huge += nr_pages;
>  		}
>  
> -- 
> 2.1.4

-- 
Michal Hocko
SUSE Labs

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