[patch 6/7] mm, memcg: do not set reclaim_state on soft limit reclaim

[akpm@linux-foundation.org]

From: Michal Hocko <mhocko@suse.com>
Subject: mm, memcg: do not set reclaim_state on soft limit reclaim

Adric Blake has noticed[1] the following warning:
[38491.963105] WARNING: CPU: 7 PID: 175 at mm/vmscan.c:245 set_task_reclaim_state+0x1e/0x40
[...]
[38491.963239] Call Trace:
[38491.963246]  mem_cgroup_shrink_node+0x9b/0x1d0
[38491.963250]  mem_cgroup_soft_limit_reclaim+0x10c/0x3a0
[38491.963254]  balance_pgdat+0x276/0x540
[38491.963258]  kswapd+0x200/0x3f0
[38491.963261]  ? wait_woken+0x80/0x80
[38491.963265]  kthread+0xfd/0x130
[38491.963267]  ? balance_pgdat+0x540/0x540
[38491.963269]  ? kthread_park+0x80/0x80
[38491.963273]  ret_from_fork+0x35/0x40
[38491.963276] ---[ end trace 727343df67b2398a ]---

which tells us that soft limit reclaim is about to overwrite the
reclaim_state configured up in the call chain (kswapd in this case but the
direct reclaim is equally possible).  This means that reclaim stats would
get misleading once the soft reclaim returns and another reclaim is done.

Fix the warning by dropping set_task_reclaim_state from the soft reclaim
which is always called with reclaim_state set up.

[1] http://lkml.kernel.org/r/CAE1jjeePxYPvw1mw2B3v803xHVR_BNnz0hQUY_JDMN8ny29M6w@mail.gmail.com

Link: http://lkml.kernel.org/r/20190828071808.20410-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Adric Blake <promarbler14@gmail.com>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hillf Danton <hdanton@sina.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vmscan.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c~mm-memcg-do-not-set-reclaim_state-on-soft-limit-reclaim
+++ a/mm/vmscan.c
@@ -3220,6 +3220,7 @@ unsigned long try_to_free_pages(struct z
 
 #ifdef CONFIG_MEMCG
 
+/* Only used by soft limit reclaim. Do not reuse for anything else. */
 unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
 						gfp_t gfp_mask, bool noswap,
 						pg_data_t *pgdat,
@@ -3235,7 +3236,8 @@ unsigned long mem_cgroup_shrink_node(str
 	};
 	unsigned long lru_pages;
 
-	set_task_reclaim_state(current, &sc.reclaim_state);
+	WARN_ON_ONCE(!current->reclaim_state);
+
 	sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
 			(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
 
@@ -3253,7 +3255,6 @@ unsigned long mem_cgroup_shrink_node(str
 
 	trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);
 
-	set_task_reclaim_state(current, NULL);
 	*nr_scanned = sc.nr_scanned;
 
 	return sc.nr_reclaimed;
_