mm: memcg: remove optimization of keeping the root_mem_cgroup LRU lists empty

root_mem_cgroup, lacking a configurable limit, was never subject to
limit reclaim, so the pages charged to it could be kept off its LRU
lists.  They would be found on the global per-zone LRU lists upon
physical memory pressure and it made sense to avoid uselessly linking
them to both lists.

The global per-zone LRU lists are about to go away on memcg-enabled
kernels, with all pages being exclusively linked to their respective
per-memcg LRU lists.  As a result, pages of the root_mem_cgroup must
also be linked to its LRU lists again.  This is purely about the LRU
list, root_mem_cgroup is still not charged.

The overhead is temporary until the double-LRU scheme is going away
completely.

Signed-off-by: Johannes Weiner <jweiner@redhat.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Ying Han <yinghan@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 10 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 750ed1449955..ad7f36f676ff 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1031,8 +1031,6 @@ void mem_cgroup_del_lru_list(struct page *page, enum lru_list lru)
         mz = page_cgroup_zoneinfo(pc->mem_cgroup, page);
         /* huge page split is done under lru_lock. so, we have no races. */
         MEM_CGROUP_ZSTAT(mz, lru) -= 1 << compound_order(page);
-        if (mem_cgroup_is_root(pc->mem_cgroup))
-                return;
         VM_BUG_ON(list_empty(&pc->lru));
         list_del_init(&pc->lru);
 }
@@ -1057,13 +1055,11 @@ void mem_cgroup_rotate_reclaimable_page(struct page *page)
                 return;
 
         pc = lookup_page_cgroup(page);
-        /* unused or root page is not rotated. */
+        /* unused page is not rotated. */
         if (!PageCgroupUsed(pc))
                 return;
         /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
         smp_rmb();
-        if (mem_cgroup_is_root(pc->mem_cgroup))
-                return;
         mz = page_cgroup_zoneinfo(pc->mem_cgroup, page);
         list_move_tail(&pc->lru, &mz->lists[lru]);
 }
@@ -1077,13 +1073,11 @@ void mem_cgroup_rotate_lru_list(struct page *page, enum lru_list lru)
                 return;
 
         pc = lookup_page_cgroup(page);
-        /* unused or root page is not rotated. */
+        /* unused page is not rotated. */
         if (!PageCgroupUsed(pc))
                 return;
         /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
         smp_rmb();
-        if (mem_cgroup_is_root(pc->mem_cgroup))
-                return;
         mz = page_cgroup_zoneinfo(pc->mem_cgroup, page);
         list_move(&pc->lru, &mz->lists[lru]);
 }
@@ -1115,8 +1109,6 @@ void mem_cgroup_add_lru_list(struct page *page, enum lru_list lru)
         /* huge page split is done under lru_lock. so, we have no races. */
         MEM_CGROUP_ZSTAT(mz, lru) += 1 << compound_order(page);
         SetPageCgroupAcctLRU(pc);
-        if (mem_cgroup_is_root(pc->mem_cgroup))
-                return;
         list_add(&pc->lru, &mz->lists[lru]);
 }