The patch titled Subject: mm/memcontrol: respect zswap.writeback setting from parent cg too has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-memcontrol-respect-zswapwriteback-setting-from-parent-cg-too.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-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Mike Yuan me@yhndnzj.com Subject: mm/memcontrol: respect zswap.writeback setting from parent cg too Date: Fri, 23 Aug 2024 16:27:06 +0000

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=3D1 still performing writeback.

The inconsistency became more noticeable after I introduced the MemoryZSwapWriteback=3D 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

Link: https://lkml.kernel.org/r/20240823162506.12117-1-me@yhndnzj.com Fixes: 501a06fe8e4c ("zswap: memcontrol: implement zswap writeback disablin= g") Signed-off-by: Mike Yuan me@yhndnzj.com Reviewed-by: Nhat Pham nphamcs@gmail.com Acked-by: Yosry Ahmed yosryahmed@google.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Michal Hocko mhocko@kernel.org Cc: Michal Koutn�� mkoutny@suse.com Cc: Muchun Song muchun.song@linux.dev Cc: Roman Gushchin roman.gushchin@linux.dev Cc: Shakeel Butt shakeel.butt@linux.dev Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

Documentation/admin-guide/cgroup-v2.rst | 7 ++++--- mm/memcontrol.c | 12 +++++++++--- 2 files changed, 13 insertions(+), 6 deletions(-)

--- a/Documentation/admin-guide/cgroup-v2.rst~mm-memcontrol-respect-zswapwriteback-setting-from-parent-cg-too +++ a/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 --- a/mm/memcontrol.c~mm-memcontrol-respect-zswapwriteback-setting-from-parent-cg-too +++ a/mm/memcontrol.c @@ -3613,8 +3613,7 @@ mem_cgroup_css_alloc(struct cgroup_subsy 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 ob 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, _

Patches currently in -mm which might be from me@yhndnzj.com are

mm-memcontrol-respect-zswapwriteback-setting-from-parent-cg-too.patch documentation-cgroup-v2-clarify-that-zswapwriteback-is-ignored-if-zswap-is-disabled.patch selftests-test_zswap-add-test-for-hierarchical-zswapwriteback.patch