On Tue 28-11-23 08:53:56, Nhat Pham wrote: > On Tue, Nov 28, 2023 at 1:38 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Mon 27-11-23 11:36:59, Nhat Pham wrote: > > > The new zswap writeback scheme requires an online-only memcg hierarchy > > > traversal. Add a new parameter to mem_cgroup_iter() to check for > > > onlineness before returning. > > > > Why is this needed? > > For context, in patch 3 of this series, Domenico and I are adding > cgroup-aware LRU to zswap, so that we can perform workload-specific > zswap writeback. When the reclaim happens due to the global zswap > limit being hit, a cgroup is selected by the mem_cgroup_iter(), and > the last one selected is saved in the zswap pool (so that the > iteration can follow from there next time the limit is hit). > > However, one problem with this scheme is we will be pinning the > reference to that saved memcg until the next global reclaim attempt, > which could prevent it from being killed for quite some time after it > has been offlined. Johannes, Yosry, and I discussed a couple of > approaches for a while, and decided to add a callback that would > release the reference held by the zswap pool when the memcg is > offlined, and the zswap pool will obtain the reference to the next > online memcg in the traversal (or at least one that has not had the > zswap-memcg-release-callback run on it yet). This should be a part of the changelog along with an explanation why this cannot be handled on the caller level? You have a pin on the memcg, you can check it is online and scratch it if not, right? Why do we need to make a rather convoluted iterator interface more complex when most users simply do not require that? -- Michal Hocko SUSE Labs
