linux-kernel - Re: [PATCH v2] mm: memcontrol: fix possible memcg leak due to interrupted reclaim

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Message-Id: <20151218144004.6ec6189817b64e04d9405001@linux-foundation.org>
Date:	Fri, 18 Dec 2015 14:40:04 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Johannes Weiner <hannes@...xchg.org>, <stable@...r.kernel.org>,
	Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: memcontrol: fix possible memcg leak due to
 interrupted reclaim

On Fri, 18 Dec 2015 19:24:05 +0300 Vladimir Davydov <vdavydov@...tuozzo.com> wrote:

> 
> OK, got it, thanks. Here goes the incremental patch (it should also fix
> the warning regarding unused cmpxchg returned value):
> ---
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fc25dc211eaf..908c075e04eb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -864,7 +864,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  			 * might block it. So we clear iter->position right
>  			 * away.
>  			 */
> -			cmpxchg(&iter->position, pos, NULL);
> +			(void)cmpxchg(&iter->position, pos, NULL);

No, this doesn't actually squish the __must_check warning.


Can anyone think of anything smarter than this?

--- a/mm/memcontrol.c~mm-memcontrol-fix-possible-memcg-leak-due-to-interrupted-reclaim-fix-fix
+++ a/mm/memcontrol.c
@@ -851,6 +851,9 @@ static struct mem_cgroup *get_mem_cgroup
 	return memcg;
 }
 
+/* Move this to compiler.h if it proves worthy */
+#define defeat_must_check(expr) do { if (expr) ; } while (0)
+
 /**
  * mem_cgroup_iter - iterate over memory cgroup hierarchy
  * @root: hierarchy root
@@ -915,7 +918,7 @@ struct mem_cgroup *mem_cgroup_iter(struc
 			 * might block it. So we clear iter->position right
 			 * away.
 			 */
-			(void)cmpxchg(&iter->position, pos, NULL);
+			defeat_must_check(cmpxchg(&iter->position, pos, NULL));
 		}
 	}
 
@@ -967,7 +970,7 @@ struct mem_cgroup *mem_cgroup_iter(struc
 		 * thread, so check that the value hasn't changed since we read
 		 * it to avoid reclaiming from the same cgroup twice.
 		 */
-		(void)cmpxchg(&iter->position, pos, memcg);
+		defeat_must_check(cmpxchg(&iter->position, pos, memcg));
 
 		/*
 		 * pairs with css_tryget when dereferencing iter->position
_

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