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...

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@cmpxchg.org 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@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 ---

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@kvack.org if you " + "depend on this functionality.\n"); + if (val & ~MOVE_MASK) return -EINVAL;

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Patches currently in -mm which might be from hannes@cmpxchg.org 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