+ mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg.patch added to -mm tree

+ mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm: memcontrol: simplify the logic of objcg pinning memcg
has been added to the -mm tree.  Its filename is
     mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg.patch

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------------------------------------------------------
From: Muchun Song <songmuchun@bytedance.com>
Subject: mm: memcontrol: simplify the logic of objcg pinning memcg

The obj_cgroup_release() and memcg_reparent_objcgs() are serialized by the
css_set_lock.  We do not need to care about objcg->memcg being released in
the process of obj_cgroup_release().  So there is no need to pin memcg
before releasing objcg.  Remove those pinning logic to simplfy the code.

There are only two places that modifies the objcg->memcg.  One is the
initialization to objcg->memcg in the memcg_online_kmem(), another is
objcgs reparenting in the memcg_reparent_objcgs().  It is also impossible
for the two to run in parallel.  So xchg() is unnecessary and it is enough
to use WRITE_ONCE().

Link: https://lkml.kernel.org/r/20210417043538.9793-7-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>
Cc: 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 |   20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

--- a/mm/memcontrol.c~mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg
+++ a/mm/memcontrol.c
@@ -261,7 +261,6 @@ static void obj_cgroup_uncharge_pages(st
 static void obj_cgroup_release(struct percpu_ref *ref)
 {
 	struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
-	struct mem_cgroup *memcg;
 	unsigned int nr_bytes;
 	unsigned int nr_pages;
 	unsigned long flags;
@@ -291,11 +290,9 @@ static void obj_cgroup_release(struct pe
 	nr_pages = nr_bytes >> PAGE_SHIFT;
 
 	spin_lock_irqsave(&css_set_lock, flags);
-	memcg = obj_cgroup_memcg(objcg);
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
 	list_del(&objcg->list);
-	mem_cgroup_put(memcg);
 	spin_unlock_irqrestore(&css_set_lock, flags);
 
 	percpu_ref_exit(ref);
@@ -330,17 +327,12 @@ static void memcg_reparent_objcgs(struct
 
 	spin_lock_irq(&css_set_lock);
 
-	/* Move active objcg to the parent's list */
-	xchg(&objcg->memcg, parent);
-	css_get(&parent->css);
-	list_add(&objcg->list, &parent->objcg_list);
-
-	/* Move already reparented objcgs to the parent's list */
-	list_for_each_entry(iter, &memcg->objcg_list, list) {
-		css_get(&parent->css);
-		xchg(&iter->memcg, parent);
-		css_put(&memcg->css);
-	}
+	/* 1) Ready to reparent active objcg. */
+	list_add(&objcg->list, &memcg->objcg_list);
+	/* 2) Reparent active objcg and already reparented objcgs to parent. */
+	list_for_each_entry(iter, &memcg->objcg_list, list)
+		WRITE_ONCE(iter->memcg, parent);
+	/* 3) Move already reparented objcgs to the parent's list */
 	list_splice(&memcg->objcg_list, &parent->objcg_list);
 
 	spin_unlock_irq(&css_set_lock);
_

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