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#Disa... [2] https://github.com/systemd/systemd/pull/31734

Changes in v3: - Additionally drop inheritance of zswap.writeback setting on cgroup creation, which is no longer needed Link to v2: https://lore.kernel.org/linux-kernel/20240816144344.18135-1-me@yhndnzj.com/

Changes in v2: - Actually base on latest tree (is_zswap_enabled() -> zswap_is_enabled()) - Update 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 Cc: Michal Koutný mkoutny@suse.com

Fixes: 501a06fe8e4c ("zswap: memcontrol: implement zswap writeback disabling") Cc: stable@vger.kernel.org

Signed-off-by: Mike Yuan me@yhndnzj.com Reviewed-by: Nhat Pham nphamcs@gmail.com Acked-by: Yosry Ahmed yosryahmed@google.com --- Documentation/admin-guide/cgroup-v2.rst | 7 ++++--- mm/memcontrol.c | 12 +++++++++--- 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst index 86311c2907cd..95c18bc17083 100644 --- a/Documentation/admin-guide/cgroup-v2.rst +++ b/Documentation/admin-guide/cgroup-v2.rst @@ -1717,9 +1717,10 @@ The following nested keys are defined. entries fault back in or are written out to disk.

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. + A read-write single value file. The default value is "1". + 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..d563fb515766 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3613,8 +3613,7 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css) memcg1_soft_limit_reset(memcg); #ifdef CONFIG_ZSWAP memcg->zswap_max = PAGE_COUNTER_MAX; - WRITE_ONCE(memcg->zswap_writeback, - !parent || READ_ONCE(parent->zswap_writeback)); + WRITE_ONCE(memcg->zswap_writeback, true); #endif page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX); if (parent) { @@ -5320,7 +5319,14 @@ 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: 47ac09b91befbb6a235ab620c32af719f8208399