Messages in this thread | | | Date | Thu, 25 May 2023 12:06:07 -0400 | Subject | Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq | From | Waiman Long <> |
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On 5/25/23 12:01, Waiman Long 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. > > 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: > > - flush blkcg per-cpu stats list in __blkg_release(), when no new stat > can be added to avoid use-after-free of the percpu blkg_iostat_set in > futue cgroup_rstat_flush*() calls. > > - add a cgroup_rstat_flush_acquire() helper and call it to acquire > cgroup_rstat_lock to block concurrent execution of other > cgroup_rstat_flush*() calls > > - don't grab bio->bi_blkg reference when adding the stats into blkcg's > per-cpu stat list since all stats are guaranteed to be consumed before > releasing blkg instance, and grabbing blkg reference for stats was > the most fragile part of original patch > > Based on Waiman's patch: > > https://lore.kernel.org/linux-block/20221215033132.230023-3-longman@redhat.com/ > > Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()") > Cc: stable@vger.kernel.org > Reported-by: Jay Shin <jaeshin@redhat.com> > Cc: Waiman Long <longman@redhat.com> > Cc: Tejun Heo <tj@kernel.org> > Cc: mkoutny@suse.com > Cc: Yosry Ahmed <yosryahmed@google.com> > Co-developed-by: Ming Lei <ming.lei@redhat.com> > Signed-off-by: Ming Lei <ming.lei@redhat.com> > Signed-off-by: Waiman Long <longman@redhat.com> > --- > block/blk-cgroup.c | 57 +++++++++++++++++++++++++++++++----------- > include/linux/cgroup.h | 1 + > kernel/cgroup/rstat.c | 15 ++++++++++- > 3 files changed, 57 insertions(+), 16 deletions(-)
This is my counter-proposal to Ming's v3 patch. The major difference is that I used the existing cgroup_rstat_lock instead of adding a new internal lock. This minimizes performance impact to existing cgroup_rstat_flush*() call while achieving the same objective. I am fine with Ming current v3 patch if we decide to go that way.
Thanks, Longman
> > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c > index 0ce64dd73cfe..90c2efc3767f 100644 > --- a/block/blk-cgroup.c > +++ b/block/blk-cgroup.c > @@ -160,13 +160,39 @@ static void blkg_free(struct blkcg_gq *blkg) > schedule_work(&blkg->free_work); > } > > +static void __blkcg_rstat_flush(struct llist_node *lnode); > + > 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, given these stat belongs to us. > + * > + * Hold the cgroup_rstat_lock before calling __blkcg_rstat_flush() > + * to block concurrent cgroup_rstat_flush*() calls. > + */ > + for_each_possible_cpu(cpu) { > + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); > + struct llist_node *lnode; > + > + if (llist_empty(lhead)) > + continue; > + > + lnode = llist_del_all(lhead); > + if (!lnode) > + continue; > + > + cgroup_rstat_flush_acquire(); > + __blkcg_rstat_flush(lnode); > + cgroup_rstat_flush_release(); > + } > > /* release the blkcg and parent blkg refs this blkg has been holding */ > css_put(&blkg->blkcg->css); > @@ -951,23 +977,12 @@ static void blkcg_iostat_update(struct blkcg_gq *blkg, struct blkg_iostat *cur, > u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags); > } > > -static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) > +static void __blkcg_rstat_flush(struct llist_node *lnode) > { > - struct blkcg *blkcg = css_to_blkcg(css); > - struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); > - struct llist_node *lnode; > struct blkg_iostat_set *bisc, *next_bisc; > > - /* Root-level stats are sourced from system-wide IO stats */ > - if (!cgroup_parent(css->cgroup)) > - return; > - > rcu_read_lock(); > > - lnode = llist_del_all(lhead); > - if (!lnode) > - goto out; > - > /* > * Iterate only the iostat_cpu's queued in the lockless list. > */ > @@ -991,13 +1006,26 @@ 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: > rcu_read_unlock(); > } > > +static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) > +{ > + struct blkcg *blkcg = css_to_blkcg(css); > + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); > + struct llist_node *lnode;Jay Shin <jaeshin@redhat.com> > + > + /* Root-level stats are sourced from system-wide IO stats */ > + if (!cgroup_parent(css->cgroup)) > + return; > + > + lnode = llist_del_all(lhead); > + if (lnode) > + __blkcg_rstat_flush(lnode); > +} > + > /* > * We source root cgroup stats from the system-wide stats to avoid > * tracking the same information twice and incurring overhead when no > @@ -2075,7 +2103,6 @@ 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..88e6647f49df 100644 > --- a/include/linux/cgroup.h > +++ b/include/linux/cgroup.h > @@ -694,6 +694,7 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu); > 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_acquire(void); > void cgroup_rstat_flush_release(void); > > /* > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c > index 9c4c55228567..b0fd4e27f466 100644 > --- a/kernel/cgroup/rstat.c > +++ b/kernel/cgroup/rstat.c > @@ -273,7 +273,20 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp) > } > > /** > - * cgroup_rstat_flush_release - release cgroup_rstat_flush_hold() > + * cgroup_rstat_flush_acquire - acquire cgroup_rstat_lock > + * > + * Callers can acquire the internal cgroup_rstat_lock to prevent concurrent > + * execution of cgroup_rstat_flush*() and the controller callbacks. > + */ > +void cgroup_rstat_flush_acquire(void) > + __acquires(&cgroup_rstat_lock) > +{ > + spin_lock_irq(&cgroup_rstat_lock); > +} > + > +/** > + * cgroup_rstat_flush_release - release cgroup_rstat_flush_hold() or > + * cgroup_rstat_flush_acquire() > */ > void cgroup_rstat_flush_release(void) > __releases(&cgroup_rstat_lock)
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