mm: memcontrol: move out cgroup swaprate throttling

The cgroup swaprate throttling is about matching new anon allocations to
the rate of available IO when that is being throttled.  It's the io
controller hooking into the VM, rather than a memory controller thing.

Rename mem_cgroup_throttle_swaprate() to cgroup_throttle_swaprate(), and
drop the @memcg argument which is only used to check whether the preceding
page charge has succeeded and the fault is proceeding.

We could decouple the call from mem_cgroup_try_charge() here as well, but
that would cause unnecessary churn: the following patches convert all
callsites to a new charge API and we'll decouple as we go along.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Link: http://lkml.kernel.org/r/20200508183105.225460-5-hannes@cmpxchg.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -3798,11 +3798,12 @@ static void free_swap_count_continuations(struct swap_info_struct *si)
 }
 
 #if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
-void mem_cgroup_throttle_swaprate(struct mem_cgroup *memcg, int node,
-				  gfp_t gfp_mask)
+void cgroup_throttle_swaprate(struct page *page, gfp_t gfp_mask)
 {
 	struct swap_info_struct *si, *next;
-	if (!(gfp_mask & __GFP_IO) || !memcg)
+	int nid = page_to_nid(page);
+
+	if (!(gfp_mask & __GFP_IO))
 		return;
 
 	if (!blk_cgroup_congested())
@@ -3816,11 +3817,10 @@ void mem_cgroup_throttle_swaprate(struct mem_cgroup *memcg, int node,
 		return;
 
 	spin_lock(&swap_avail_lock);
-	plist_for_each_entry_safe(si, next, &swap_avail_heads[node],
-				  avail_lists[node]) {
+	plist_for_each_entry_safe(si, next, &swap_avail_heads[nid],
+				  avail_lists[nid]) {
 		if (si->bdev) {
-			blkcg_schedule_throttle(bdev_get_queue(si->bdev),
-						true);
+			blkcg_schedule_throttle(bdev_get_queue(si->bdev), true);
 			break;
 		}
 	}