[PATCH v2 1/2] mm/memcontrol: respect zswap.writeback setting from parent cg too

[Mike Yuan <me@yhndnzj.com>]

Currently, the behavior of zswap.writeback wrt.
the cgroup hierarchy seems a bit odd. Unlike zswap.max,
it doesn't honor the value from parent cgroups. This
surfaced when people tried to globally disable zswap writeback,
i.e. reserve physical swap space only for hibernation [1] -
disabling zswap.writeback only for the root cgroup results
in subcgroups with zswap.writeback=1 still performing writeback.

The inconsistency became more noticeable after I introduced
the MemoryZSwapWriteback= systemd unit setting [2] for
controlling the knob. The patch assumed that the kernel would
enforce the value of parent cgroups. It could probably be
workarounded from systemd's side, by going up the slice unit
tree and inheriting the value. Yet I think it's more sensible
to make it behave consistently with zswap.max and friends.

[1] https://wiki.archlinux.org/title/Power_management/Suspend_and_hibernate#Disable_zswap_writeback_to_use_the_swap_space_only_for_hibernation
[2] https://github.com/systemd/systemd/pull/31734

Changes in v2:
- Actually base on latest tree (is_zswap_enabled() -> zswap_is_enabled())
- Updated Documentation/admin-guide/cgroup-v2.rst to reflect the change

Link to v1: https://lore.kernel.org/linux-kernel/20240814171800.23558-1-me@yhndnzj.com/

Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Mike Yuan <me@yhndnzj.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
---
 Documentation/admin-guide/cgroup-v2.rst | 5 ++++-
 mm/memcontrol.c                         | 9 ++++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 86311c2907cd..80906cea4264 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1719,7 +1719,10 @@ The following nested keys are defined.
   memory.zswap.writeback
 	A read-write single value file. The default value is "1". The
 	initial value of the root cgroup is 1, and when a new cgroup is
-	created, it inherits the current value of its parent.
+	created, it inherits the current value of its parent. Note that
+	this setting is hierarchical, i.e. the writeback would be
+	implicitly disabled for child cgroups if the upper hierarchy
+	does so.
 
 	When this is set to 0, all swapping attempts to swapping devices
 	are disabled. This included both zswap writebacks, and swapping due
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f29157288b7d..327b2b030639 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5320,7 +5320,14 @@ void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size)
 bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
 {
 	/* if zswap is disabled, do not block pages going to the swapping device */
-	return !zswap_is_enabled() || !memcg || READ_ONCE(memcg->zswap_writeback);
+	if (!zswap_is_enabled())
+		return true;
+
+	for (; memcg; memcg = parent_mem_cgroup(memcg))
+		if (!READ_ONCE(memcg->zswap_writeback))
+			return false;
+
+	return true;
 }
 
 static u64 zswap_current_read(struct cgroup_subsys_state *css,

base-commit: d07b43284ab356daf7ec5ae1858a16c1c7b6adab
-- 
2.46.0