The patch titled
Subject: mm: memcontrol: deprecate charge moving
has been added to the -mm mm-unstable branch. Its filename is
mm-memcontrol-deprecate-charge-moving.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memcontrol-deprecate-charge-moving.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: mm: memcontrol: deprecate charge moving
Date: Wed, 7 Dec 2022 14:00:39 +0100
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.
Link: https://lkml.kernel.org/r/Y5COd+qXwk/S+n8N@xxxxxxxxxxx
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/cgroup-v1/memory.rst | 11 ++++++++++-
mm/memcontrol.c | 4 ++++
2 files changed, 14 insertions(+), 1 deletion(-)
--- a/Documentation/admin-guide/cgroup-v1/memory.rst~mm-memcontrol-deprecate-charge-moving
+++ a/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
@@ -717,9 +719,16 @@ 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.
This feature is not supported in !CONFIG_MMU environments because of lack of
--- a/mm/memcontrol.c~mm-memcontrol-deprecate-charge-moving
+++ a/mm/memcontrol.c
@@ -3919,6 +3919,10 @@ static int mem_cgroup_move_charge_write(
{
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@xxxxxxxxx if you "
+ "depend on this functionality.\n");
+
if (val & ~MOVE_MASK)
return -EINVAL;
_
Patches currently in -mm which might be from hannes@xxxxxxxxxxx are
mm-memcontrol-skip-moving-non-present-pages-that-are-mapped-elsewhere.patch
mm-rmap-remove-lock_page_memcg.patch
mm-memcontrol-deprecate-charge-moving.patch
