[PATCH] mm: memcontrol: clarify the uncharge_list() loop

[PATCH] mm: memcontrol: clarify the uncharge_list() loop

[Johannes Weiner]

uncharge_list() does an unusual list walk because the function can
take regular lists with dedicated list_heads as well as singleton
lists where a single page is passed via the page->lru list node.

This can sometimes lead to confusion as well as suggestions to replace
the loop with a list_for_each_entry(), which wouldn't work.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memcontrol.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8614e0d750e5..fa7bf354ae32 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5420,6 +5420,10 @@ static void uncharge_list(struct list_head *page_list)
 	struct list_head *next;
 	struct page *page;
 
+	/*
+	 * Note that the list can be a single page->lru; hence the
+	 * do-while loop instead of a simple list_for_each_entry().
+	 */
 	next = page_list->next;
 	do {
 		unsigned int nr_pages = 1;
-- 
2.7.2

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Re: [PATCH] mm: memcontrol: clarify the uncharge_list() loop

[Andrew Morton]

LGTM.  It's very late in the cycle so I'll queue all three for 4.6-rc1
and I tagged the first two patches (not this one) for -stable
backporting.

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Re: [PATCH] mm: memcontrol: clarify the uncharge_list() loop

[Johannes Weiner]

On Thu, Mar 10, 2016 at 01:30:34PM -0800, Andrew Morton wrote:
> 
> LGTM.  It's very late in the cycle so I'll queue all three for 4.6-rc1
> and I tagged the first two patches (not this one) for -stable
> backporting.

Sounds good, thank you.

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Re: [PATCH] mm: memcontrol: clarify the uncharge_list() loop

[Michal Hocko]

On Thu 10-03-16 15:50:15, Johannes Weiner wrote:
> uncharge_list() does an unusual list walk because the function can
> take regular lists with dedicated list_heads as well as singleton
> lists where a single page is passed via the page->lru list node.
> 
> This can sometimes lead to confusion as well as suggestions to replace
> the loop with a list_for_each_entry(), which wouldn't work.

Yes this confused at least me 2 times AFAIR.

> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/memcontrol.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8614e0d750e5..fa7bf354ae32 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5420,6 +5420,10 @@ static void uncharge_list(struct list_head *page_list)
>  	struct list_head *next;
>  	struct page *page;
>  
> +	/*
> +	 * Note that the list can be a single page->lru; hence the
> +	 * do-while loop instead of a simple list_for_each_entry().
> +	 */
>  	next = page_list->next;
>  	do {
>  		unsigned int nr_pages = 1;
> -- 
> 2.7.2

-- 
Michal Hocko
SUSE Labs

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