mm/vmscan.c: add checks for incorrect handling of current->reclaim_state

Six sites are presently altering current->reclaim_state.  There is a
risk that one function stomps on a caller's value.  Use a helper
function to catch such errors.

Cc: Yafang Shao <laoar.shao@gmail.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -241,6 +241,18 @@ static void unregister_memcg_shrinker(struct shrinker *shrinker)
 }
 #endif /* CONFIG_MEMCG_KMEM */
 
+static void set_task_reclaim_state(struct task_struct *task,
+				   struct reclaim_state *rs)
+{
+	/* Check for an overwrite */
+	WARN_ON_ONCE(rs && task->reclaim_state);
+
+	/* Check for the nulling of an already-nulled member */
+	WARN_ON_ONCE(!rs && !task->reclaim_state);
+
+	task->reclaim_state = rs;
+}
+
 #ifdef CONFIG_MEMCG
 static bool global_reclaim(struct scan_control *sc)
 {
@@ -3194,13 +3206,13 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 	if (throttle_direct_reclaim(sc.gfp_mask, zonelist, nodemask))
 		return 1;
 
-	current->reclaim_state = &sc.reclaim_state;
+	set_task_reclaim_state(current, &sc.reclaim_state);
 	trace_mm_vmscan_direct_reclaim_begin(order, sc.gfp_mask);
 
 	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
 
 	trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
-	current->reclaim_state = NULL;
+	set_task_reclaim_state(current, NULL);
 
 	return nr_reclaimed;
 }
@@ -3223,7 +3235,7 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
 	};
 	unsigned long lru_pages;
 
-	current->reclaim_state = &sc.reclaim_state;
+	set_task_reclaim_state(current, &sc.reclaim_state);
 	sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
 			(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
 
@@ -3241,7 +3253,7 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
 
 	trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);
 
-	current->reclaim_state = NULL;
+	set_task_reclaim_state(current, NULL);
 	*nr_scanned = sc.nr_scanned;
 
 	return sc.nr_reclaimed;
@@ -3270,7 +3282,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 		.may_shrinkslab = 1,
 	};
 
-	current->reclaim_state = &sc.reclaim_state;
+	set_task_reclaim_state(current, &sc.reclaim_state);
 	/*
 	 * Unlike direct reclaim via alloc_pages(), memcg's reclaim doesn't
 	 * take care of from where we get pages. So the node where we start the
@@ -3291,7 +3303,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 	psi_memstall_leave(&pflags);
 
 	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
-	current->reclaim_state = NULL;
+	set_task_reclaim_state(current, NULL);
 
 	return nr_reclaimed;
 }
@@ -3493,7 +3505,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 		.may_unmap = 1,
 	};
 
-	current->reclaim_state = &sc.reclaim_state;
+	set_task_reclaim_state(current, &sc.reclaim_state);
 	psi_memstall_enter(&pflags);
 	__fs_reclaim_acquire();
 
@@ -3675,7 +3687,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 	snapshot_refaults(NULL, pgdat);
 	__fs_reclaim_release();
 	psi_memstall_leave(&pflags);
-	current->reclaim_state = NULL;
+	set_task_reclaim_state(current, NULL);
 
 	/*
 	 * Return the order kswapd stopped reclaiming at as
@@ -3940,17 +3952,16 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
 		.hibernation_mode = 1,
 	};
 	struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
-	struct task_struct *p = current;
 	unsigned long nr_reclaimed;
 	unsigned int noreclaim_flag;
 
 	fs_reclaim_acquire(sc.gfp_mask);
 	noreclaim_flag = memalloc_noreclaim_save();
-	p->reclaim_state = &sc.reclaim_state;
+	set_task_reclaim_state(current, &sc.reclaim_state);
 
 	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
 
-	p->reclaim_state = NULL;
+	set_task_reclaim_state(current, NULL);
 	memalloc_noreclaim_restore(noreclaim_flag);
 	fs_reclaim_release(sc.gfp_mask);
 
@@ -4139,7 +4150,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	 */
 	noreclaim_flag = memalloc_noreclaim_save();
 	p->flags |= PF_SWAPWRITE;
-	p->reclaim_state = &sc.reclaim_state;
+	set_task_reclaim_state(p, &sc.reclaim_state);
 
 	if (node_pagecache_reclaimable(pgdat) > pgdat->min_unmapped_pages) {
 		/*
@@ -4151,7 +4162,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 		} while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
 	}
 
-	p->reclaim_state = NULL;
+	set_task_reclaim_state(p, NULL);
 	current->flags &= ~PF_SWAPWRITE;
 	memalloc_noreclaim_restore(noreclaim_flag);
 	fs_reclaim_release(sc.gfp_mask);