+ mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm.patch added to -mm tree

+ mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm
has been added to the -mm tree.  Its filename is
     mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm.patch

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------------------------------------------------------
From: Muchun Song <songmuchun@bytedance.com>
Subject: mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm

When mm is NULL, we do not need to hold rcu lock and call css_tryget for
the root memcg.  And we also do not need to check !mm in every loop of
while.  So bail out early when !mm.

Link: https://lkml.kernel.org/r/20210417043538.9793-3-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memcontrol.c |   25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

--- a/mm/memcontrol.c~mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm
+++ a/mm/memcontrol.c
@@ -919,20 +919,23 @@ struct mem_cgroup *get_mem_cgroup_from_m
 	if (mem_cgroup_disabled())
 		return NULL;
 
+	/*
+	 * Page cache insertions can happen without an
+	 * actual mm context, e.g. during disk probing
+	 * on boot, loopback IO, acct() writes etc.
+	 *
+	 * No need to css_get on root memcg as the reference
+	 * counting is disabled on the root level in the
+	 * cgroup core. See CSS_NO_REF.
+	 */
+	if (unlikely(!mm))
+		return root_mem_cgroup;
+
 	rcu_read_lock();
 	do {
-		/*
-		 * Page cache insertions can happen without an
-		 * actual mm context, e.g. during disk probing
-		 * on boot, loopback IO, acct() writes etc.
-		 */
-		if (unlikely(!mm))
+		memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
+		if (unlikely(!memcg))
 			memcg = root_mem_cgroup;
-		else {
-			memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
-			if (unlikely(!memcg))
-				memcg = root_mem_cgroup;
-		}
 	} while (!css_tryget(&memcg->css));
 	rcu_read_unlock();
 	return memcg;
_

Patches currently in -mm which might be from songmuchun@bytedance.com are

mm-memcontrol-fix-root_mem_cgroup-charging.patch
mm-memcontrol-fix-page-charging-in-page-replacement.patch
mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm.patch
mm-memcontrol-remove-the-pgdata-parameter-of-mem_cgroup_page_lruvec.patch
mm-memcontrol-simplify-lruvec_holds_page_lru_lock.patch
mm-memcontrol-rename-lruvec_holds_page_lru_lock-to-page_matches_lruvec.patch
mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg.patch
mm-memcontrol-move-obj_cgroup_uncharge_pages-out-of-css_set_lock.patch
mm-vmscan-remove-noinline_for_stack.patch
mm-memory_hotplug-factor-out-bootmem-core-functions-to-bootmem_infoc.patch
mm-hugetlb-introduce-a-new-config-hugetlb_page_free_vmemmap.patch
mm-hugetlb-gather-discrete-indexes-of-tail-page.patch
mm-hugetlb-free-the-vmemmap-pages-associated-with-each-hugetlb-page.patch
mm-hugetlb-defer-freeing-of-hugetlb-pages.patch
mm-hugetlb-alloc-the-vmemmap-pages-associated-with-each-hugetlb-page.patch
mm-hugetlb-add-a-kernel-parameter-hugetlb_free_vmemmap.patch
mm-memory_hotplug-disable-memmap_on_memory-when-hugetlb_free_vmemmap-enabled.patch
mm-hugetlb-introduce-nr_free_vmemmap_pages-in-the-struct-hstate.patch