On Fri, Aug 16, 2024 at 7:44 AM Mike Yuan <me@xxxxxxxxxxx> wrote: > > 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@xxxxxxxxxxx/ > > Cc: Nhat Pham <nphamcs@xxxxxxxxx> > Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Mike Yuan <me@xxxxxxxxxxx> > Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx> LGTM, Acked-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > --- > 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; This is orthogonal to this patch, but I just realized that we completely ignore memory.zswap_writeback if zswap is disabled. This means that if a cgroup has disabled writeback, then zswap is globally disabled for some reason, we stop respecting the cgroup knob. I guess the rationale could be that we want to help get pages out of zswap as much as possible to honor zswap's disablement? Nhat, did I get that right? I feel like it's a little bit odd to be honest, but I don't have a strong opinion on it. Maybe we should document this behavior better. > + > + 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 > >
