From: Johannes Weiner <hannes@cmpxchg.org>

Date: Wed, 7 Dec 2022 14:00:39 +0100

Subject: mm: memcontrol: deprecate charge moving

Git-commit: da34a8484d162585e22ed8c1e4114aa2f60e3567

Patch-mainline: v6.3-rc1

References: bsc#1209801

Charge moving mode in cgroup1 allows memory to follow tasks as they

migrate between cgroups.  This is, and always has been, a questionable

thing to do - for several reasons.

First, it's expensive.  Pages need to be identified, locked and isolated

from various MM operations, and reassigned, one by one.

Second, it's unreliable.  Once pages are charged to a cgroup, there isn't

always a clear owner task anymore.  Cache isn't moved at all, for example.

Mapped memory is moved - but if trylocking or isolating a page fails,

it's arbitrarily left behind.  Frequent moving between domains may leave a

task's memory scattered all over the place.

Third, it isn't really needed.  Launcher tasks can kick off workload tasks

directly in their target cgroup.  Using dedicated per-workload groups

allows fine-grained policy adjustments - no need to move tasks and their

physical pages between control domains.  The feature was never

forward-ported to cgroup2, and it hasn't been missed.

Despite it being a niche usecase, the maintenance overhead of supporting

it is enormous.  Because pages are moved while they are live and subject

to various MM operations, the synchronization rules are complicated.

There are lock_page_memcg() in MM and FS code, which non-cgroup people

don't understand.  In some cases we've been able to shift code and cgroup

API calls around such that we can rely on native locking as much as

possible.  But that's fragile, and sometimes we need to hold MM locks for

longer than we otherwise would (pte lock e.g.).

Mark the feature deprecated. Hopefully we can remove it soon.

And backport into -stable kernels so that people who develop against

earlier kernels are warned about this deprecation as early as possible.

[akpm@linux-foundation.org: fix memory.rst underlining]

Link: https://lkml.kernel.org/r/Y5COd+qXwk/S+n8N@cmpxchg.org

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Shakeel Butt <shakeelb@google.com>

Acked-by: Hugh Dickins <hughd@google.com>

Acked-by: Michal Hocko <mhocko@suse.com>

Cc: Muchun Song <songmuchun@bytedance.com>

Cc: Roman Gushchin <roman.gushchin@linux.dev>

Cc: <stable@vger.kernel.org>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Michal Koutný <mkoutny@suse.com>

---

 Documentation/admin-guide/cgroup-v1/memory.rst | 13 +++++++++++--

 mm/memcontrol.c                                |  4 ++++

 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst

index 60370f2c67b9..258e45cc3b2d 100644

--- a/Documentation/admin-guide/cgroup-v1/memory.rst

+++ b/Documentation/admin-guide/cgroup-v1/memory.rst

@@ -86,6 +86,8 @@ Brief summary of control files.

  memory.swappiness		     set/show swappiness parameter of vmscan

 				     (See sysctl's vm.swappiness)

  memory.move_charge_at_immigrate     set/show controls of moving charges

+                                     This knob is deprecated and shouldn't be

+                                     used.

  memory.oom_control		     set/show oom controls.

  memory.numa_stat		     show the number of memory usage per numa

 				     node

@@ -717,8 +719,15 @@ NOTE2:

        It is recommended to set the soft limit always below the hard limit,

        otherwise the hard limit will take precedence.

-8. Move charges at task migration

-=================================

+8. Move charges at task migration (DEPRECATED!)

+===============================================

+

+THIS IS DEPRECATED!

+

+It's expensive and unreliable! It's better practice to launch workload

+tasks directly from inside their target cgroup. Use dedicated workload

+cgroups to allow fine-grained policy adjustments without having to

+move physical pages between control domains.

 Users can move charges associated with a task along with task migration, that

 is, uncharge task's pages from the old cgroup and charge them to the new cgroup.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c

index a698a2b6523b..49f67176a1a2 100644

--- a/mm/memcontrol.c

+++ b/mm/memcontrol.c

@@ -3919,6 +3919,10 @@ static int mem_cgroup_move_charge_write(struct cgroup_subsys_state *css,

 {

 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);

+	pr_warn_once("Cgroup memory moving (move_charge_at_immigrate) is deprecated. "

+		     "Please report your usecase to linux-mm@kvack.org if you "

+		     "depend on this functionality.\n");

+

 	if (val & ~MOVE_MASK)

 		return -EINVAL;