This is a note to let you know that I've just added the patch titled
mm: memcontrol: decouple reference counting from page accounting
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-memcontrol-decouple-reference-counting-from-page-accounting.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 1a3e1f40962c445b997151a542314f3c6097f8c3 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Date: Thu, 6 Aug 2020 23:20:45 -0700
Subject: mm: memcontrol: decouple reference counting from page accounting
From: Johannes Weiner <hannes@xxxxxxxxxxx>
commit 1a3e1f40962c445b997151a542314f3c6097f8c3 upstream.
The reference counting of a memcg is currently coupled directly to how
many 4k pages are charged to it. This doesn't work well with Roman's new
slab controller, which maintains pools of objects and doesn't want to keep
an extra balance sheet for the pages backing those objects.
This unusual refcounting design (reference counts usually track pointers
to an object) is only for historical reasons: memcg used to not take any
css references and simply stalled offlining until all charges had been
reparented and the page counters had dropped to zero. When we got rid of
the reparenting requirement, the simple mechanical translation was to take
a reference for every charge.
More historical context can be found in commit e8ea14cc6ead ("mm:
memcontrol: take a css reference for each charged page"), commit
64f219938941 ("mm: memcontrol: remove obsolete kmemcg pinning tricks") and
commit b2052564e66d ("mm: memcontrol: continue cache reclaim from offlined
groups").
The new slab controller exposes the limitations in this scheme, so let's
switch it to a more idiomatic reference counting model based on actual
kernel pointers to the memcg:
- The per-cpu stock holds a reference to the memcg its caching
- User pages hold a reference for their page->mem_cgroup. Transparent
huge pages will no longer acquire tail references in advance, we'll
get them if needed during the split.
- Kernel pages hold a reference for their page->mem_cgroup
- Pages allocated in the root cgroup will acquire and release css
references for simplicity. css_get() and css_put() optimize that.
- The current memcg_charge_slab() already hacked around the per-charge
references; this change gets rid of that as well.
- tcp accounting will handle reference in mem_cgroup_sk_{alloc,free}
Roman:
1) Rebased on top of the current mm tree: added css_get() in
mem_cgroup_charge(), dropped mem_cgroup_try_charge() part
2) I've reformatted commit references in the commit log to make
checkpatch.pl happy.
[hughd@xxxxxxxxxx: remove css_put_many() from __mem_cgroup_clear_mc()]
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2007302011450.2347@eggly.anvils
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Link: http://lkml.kernel.org/r/20200623174037.3951353-6-guro@xxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Fixes: cdec2e4265df ("memcg: coalesce charging via percpu storage")
Signed-off-by: GONG, Ruiqi <gongruiqi1@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2214,6 +2214,9 @@ static void drain_stock(struct memcg_sto
{
struct mem_cgroup *old = stock->cached;
+ if (!old)
+ return;
+
if (stock->nr_pages) {
page_counter_uncharge(&old->memory, stock->nr_pages);
if (do_memsw_account())
@@ -2221,6 +2224,8 @@ static void drain_stock(struct memcg_sto
css_put_many(&old->css, stock->nr_pages);
stock->nr_pages = 0;
}
+
+ css_put(&old->css);
stock->cached = NULL;
}
@@ -2256,6 +2261,7 @@ static void refill_stock(struct mem_cgro
stock = this_cpu_ptr(&memcg_stock);
if (stock->cached != memcg) { /* reset if necessary */
drain_stock(stock);
+ css_get(&memcg->css);
stock->cached = memcg;
}
stock->nr_pages += nr_pages;
Patches currently in stable-queue which might be from hannes@xxxxxxxxxxx are
queue-5.4/mm-memcontrol-decouple-reference-counting-from-page-accounting.patch
