On Fri, Mar 13, 2026 at 12:40:01PM +0100, Natalie Vock wrote: > This helps to find a common subtree of two resources, which is important > when determining whether it's helpful to evict one resource in favor of > another. > > To facilitate this, add a common helper to find the ancestor of two > cgroups using each cgroup's ancestor array. > > Signed-off-by: Natalie Vock <[email protected]> > --- > include/linux/cgroup.h | 21 +++++++++++++++++++++ > include/linux/cgroup_dmem.h | 9 +++++++++ > kernel/cgroup/dmem.c | 28 ++++++++++++++++++++++++++++ > 3 files changed, 58 insertions(+) > > diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h > index bc892e3b37eea..560ae995e3a54 100644 > --- a/include/linux/cgroup.h > +++ b/include/linux/cgroup.h > @@ -561,6 +561,27 @@ static inline struct cgroup *cgroup_ancestor(struct > cgroup *cgrp, > return cgrp->ancestors[ancestor_level]; > } > > +/** > + * cgroup_common_ancestor - find common ancestor of two cgroups > + * @a: first cgroup to find common ancestor of > + * @b: second cgroup to find common ancestor of > + * > + * Find the first cgroup that is an ancestor of both @a and @b, if it exists > + * and return a pointer to it. If such a cgroup doesn't exist, return NULL. > + * > + * This function is safe to call as long as both @a and @b are accessible. > + */ > +static inline struct cgroup *cgroup_common_ancestor(struct cgroup *a, > + struct cgroup *b) > +{ > + int level; > + > + for (level = min(a->level, b->level); level >= 0; level--) > + if (a->ancestors[level] == b->ancestors[level]) > + return a->ancestors[level]; > + return NULL; > +} > + > /** > * task_under_cgroup_hierarchy - test task's membership of cgroup ancestry > * @task: the task to be tested > diff --git a/include/linux/cgroup_dmem.h b/include/linux/cgroup_dmem.h > index 1a88cd0c9eb00..9d72457c4cb9d 100644 > --- a/include/linux/cgroup_dmem.h > +++ b/include/linux/cgroup_dmem.h > @@ -28,6 +28,8 @@ bool dmem_cgroup_below_min(struct dmem_cgroup_pool_state > *root, > struct dmem_cgroup_pool_state *test); > bool dmem_cgroup_below_low(struct dmem_cgroup_pool_state *root, > struct dmem_cgroup_pool_state *test); > +struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct > dmem_cgroup_pool_state *a, > + struct > dmem_cgroup_pool_state *b); > > void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_state *pool); > #else > @@ -75,6 +77,13 @@ static inline bool dmem_cgroup_below_low(struct > dmem_cgroup_pool_state *root, > return false; > } > > +static inline > +struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct > dmem_cgroup_pool_state *a, > + struct > dmem_cgroup_pool_state *b) > +{ > + return NULL; > +} > + > static inline void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_state > *pool) > { } > > diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c > index 28227405f7cfe..9ae085a7fcb73 100644 > --- a/kernel/cgroup/dmem.c > +++ b/kernel/cgroup/dmem.c > @@ -756,6 +756,34 @@ bool dmem_cgroup_below_low(struct dmem_cgroup_pool_state > *root, > } > EXPORT_SYMBOL_GPL(dmem_cgroup_below_low); > > +/** > + * dmem_cgroup_get_common_ancestor(): Find the first common ancestor of two > pools. > + * @a: First pool to find the common ancestor of. > + * @b: First pool to find the common ancestor of. > + * > + * Return: The first pool that is a parent of both @a and @b, or NULL if > either @a or @b are NULL, > + * or if such a pool does not exist. A reference to the returned pool is > grabbed and must be > + * released by the caller when it is done using the pool. > + */ > +struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct > dmem_cgroup_pool_state *a, > + struct > dmem_cgroup_pool_state *b) > +{ > + struct cgroup *ancestor_cgroup; > + struct cgroup_subsys_state *ancestor_css; > + > + if (!a || !b) > + return NULL; > + > + ancestor_cgroup = cgroup_common_ancestor(a->cs->css.cgroup, > b->cs->css.cgroup); > + if (!ancestor_cgroup) > + return NULL; > + > + ancestor_css = cgroup_e_css(ancestor_cgroup, &dmem_cgrp_subsys);
cgroup_e_css must be called in RCU read context. Besides, a reference to ancestor_css must be got as later on, dmem_cgroup_pool_state_put will call css_put. Here is my fixup, which I tested and did not cause RCU or reference warnings whereas the original patch caused such issues. Feel free to use it with my sign-off. Signed-off-by: Thadeu Lima de Souza Cascardo <[email protected]> Regards. Cascardo. diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c index 72ee8f1d69ef..28adb042baca 100644 --- a/kernel/cgroup/dmem.c +++ b/kernel/cgroup/dmem.c @@ -773,6 +773,7 @@ struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct dmem_cgrou { struct cgroup *ancestor_cgroup; struct cgroup_subsys_state *ancestor_css; + struct dmem_cgroup_pool_state *pool = NULL; if (!a || !b) return NULL; @@ -781,9 +782,15 @@ struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct dmem_cgrou if (!ancestor_cgroup) return NULL; + rcu_read_lock(); ancestor_css = cgroup_e_css(ancestor_cgroup, &dmem_cgrp_subsys); + if (css_tryget(ancestor_css)) + pool = get_cg_pool_unlocked(css_to_dmemcs(ancestor_css), a->region); + if (!pool) + css_put(ancestor_css); + rcu_read_unlock(); - return get_cg_pool_unlocked(css_to_dmemcs(ancestor_css), a->region); + return pool; } EXPORT_SYMBOL_GPL(dmem_cgroup_get_common_ancestor); -- 2.47.3 > + > + return get_cg_pool_unlocked(css_to_dmemcs(ancestor_css), a->region); > +} > +EXPORT_SYMBOL_GPL(dmem_cgroup_get_common_ancestor); > + > static int dmem_cgroup_region_capacity_show(struct seq_file *sf, void *v) > { > struct dmem_cgroup_region *region; > > -- > 2.53.0 >
