Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq

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.c...

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)