Hi Ming, On Tue, May 23, 2023 at 6:21 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > As noted by Michal, the blkg_iostat_set's in the lockless list > hold reference to blkg's to protect against their removal. Those > blkg's hold reference to blkcg. When a cgroup is being destroyed, > cgroup_rstat_flush() is only called at css_release_work_fn() which > is called when the blkcg reference count reaches 0. This circular > dependency will prevent blkcg and some blkgs from being freed after > they are made offline. I am not at all familiar with blkcg, but does calling cgroup_rstat_flush() in offline_css() fix the problem? or can items be added to the lockless list(s) after the blkcg is offlined? > > It is less a problem if the cgroup to be destroyed also has other > controllers like memory that will call cgroup_rstat_flush() which will > clean up the reference count. If block is the only controller that uses > rstat, these offline blkcg and blkgs may never be freed leaking more > and more memory over time. > > To prevent this potential memory leak: > > - a new cgroup_rstat_css_cpu_flush() function is added to flush stats for > a given css and cpu. This new function will be called in __blkg_release(). > > - don't grab bio->bi_blkg when adding the stats into blkcg's per-cpu > stat list, and this kind of handling is the most fragile part of > original patch > > Based on Waiman's patch: > > https://lore.kernel.org/linux-block/20221215033132.230023-3-longman@xxxxxxxxxx/ > > Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()") > Cc: Waiman Long <longman@xxxxxxxxxx> > Cc: cgroups@xxxxxxxxxxxxxxx > Cc: Tejun Heo <tj@xxxxxxxxxx> > Cc: mkoutny@xxxxxxxx > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> > --- > block/blk-cgroup.c | 15 +++++++++++++-- > include/linux/cgroup.h | 1 + > kernel/cgroup/rstat.c | 18 ++++++++++++++++++ > 3 files changed, 32 insertions(+), 2 deletions(-) > > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c > index 0ce64dd73cfe..5437b6af3955 100644 > --- a/block/blk-cgroup.c > +++ b/block/blk-cgroup.c > @@ -163,10 +163,23 @@ static void blkg_free(struct blkcg_gq *blkg) > static void __blkg_release(struct rcu_head *rcu) > { > struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head); > + struct blkcg *blkcg = blkg->blkcg; > + int cpu; > > #ifdef CONFIG_BLK_CGROUP_PUNT_BIO > WARN_ON(!bio_list_empty(&blkg->async_bios)); > #endif > + /* > + * Flush all the non-empty percpu lockless lists before releasing > + * us. Meantime no new bio can refer to this blkg any more given > + * the refcnt is killed. > + */ > + for_each_possible_cpu(cpu) { > + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); > + > + if (!llist_empty(lhead)) > + cgroup_rstat_css_cpu_flush(&blkcg->css, cpu); > + } > > /* release the blkcg and parent blkg refs this blkg has been holding */ > css_put(&blkg->blkcg->css); > @@ -991,7 +1004,6 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) > if (parent && parent->parent) > blkcg_iostat_update(parent, &blkg->iostat.cur, > &blkg->iostat.last); > - percpu_ref_put(&blkg->refcnt); > } > > out: > @@ -2075,7 +2087,6 @@ void blk_cgroup_bio_start(struct bio *bio) > > llist_add(&bis->lnode, lhead); > WRITE_ONCE(bis->lqueued, true); > - percpu_ref_get(&bis->blkg->refcnt); > } > > u64_stats_update_end_irqrestore(&bis->sync, flags); > diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h > index 885f5395fcd0..97d4764d8e6a 100644 > --- a/include/linux/cgroup.h > +++ b/include/linux/cgroup.h > @@ -695,6 +695,7 @@ void cgroup_rstat_flush(struct cgroup *cgrp); > void cgroup_rstat_flush_atomic(struct cgroup *cgrp); > void cgroup_rstat_flush_hold(struct cgroup *cgrp); > void cgroup_rstat_flush_release(void); > +void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu); > > /* > * Basic resource stats. > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c > index 9c4c55228567..96e7a4e6da72 100644 > --- a/kernel/cgroup/rstat.c > +++ b/kernel/cgroup/rstat.c > @@ -281,6 +281,24 @@ void cgroup_rstat_flush_release(void) > spin_unlock_irq(&cgroup_rstat_lock); > } > > +/** > + * cgroup_rstat_css_cpu_flush - flush stats for the given css and cpu > + * @css: target css to be flush > + * @cpu: the cpu that holds the stats to be flush > + * > + * A lightweight rstat flush operation for a given css and cpu. > + * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock > + * isn't used. (Adding linux-mm and memcg maintainers) +Linux-MM +Michal Hocko +Shakeel Butt +Johannes Weiner +Roman Gushchin +Muchun Song I don't think flushing the stats without holding cgroup_rstat_lock is safe for memcg stats flushing. mem_cgroup_css_rstat_flush() modifies some non-percpu data (e.g. memcg->vmstats->state, memcg->vmstats->state_pending). Perhaps have this be a separate callback than css_rstat_flush() (e.g. css_rstat_flush_cpu() or something), so that it's clear what subsystems support this? In this case, only blkcg would implement this callback. > + */ > +void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu) > +{ > + raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu); > + > + raw_spin_lock_irq(cpu_lock); > + css->ss->css_rstat_flush(css, cpu); I think we need to check that css_rstat_flush() (or a new callback) is implemented before calling it here. > + raw_spin_unlock_irq(cpu_lock); > +} > + > int cgroup_rstat_init(struct cgroup *cgrp) > { > int cpu; > -- > 2.40.1 >
