Re: [PATCH] bfq: fix blkio cgroup leakage

[Dmitry Monakhov <dmonakhov@gmail.com>]

Paolo Valente <paolo.valente@linaro.org> writes:

> Hi,
> sorry for the delay.  The commit you propose to drop fix the issues
> reported in [1].
>
> Such a commit does introduce the leak that you report (thank you for
> spotting it).  Yet, according to the threads mentioned in [1],
> dropping that commit would take us back to those issues.
>
> Maybe the solution is to fix the unbalance that you spotted?
I'm not quite shure that do I understand which bug was addressed for commit db37a34c563b.
AFAIU both bugs mentioned in original patchset was fixed by:
478de3380 ("block, bfq: deschedule empty bfq_queues not referred by any proces")
f718b0932 ( block, bfq: do not plug I/O for bfq_queues with no proc refs)"

So I review commit db37a34c563b as independent one.
It introduces extra reference for bfq_groups via bfqg_and_blkg_get(),
but do we actually need it here?

#IF CONFIG_BFQ_GROUP_IOSCHED is enabled:
 bfqd->root_group is holded by bfqd from bfq_init_queue()
 other bfq_queue objects are owned by corresponding blkcg from bfq_pd_alloc()
 So bfq_queue can not disappear under us.
 
#IF CONFIG_BFQ_GROUP_IOSCHED is disabled:
 we have only one  bfqd->root_group object which allocated from bfq_create_group_hierarch()
 and bfqg_and_blkg_get() bfqg_and_blkg_put() are noop

Resume: in both cases extra reference is not required, so I continue to
insist that we should revert  commit db37a34c563b because it tries to
solve a non existing issue, but introduce the real one.

Please correct me if I'm wrong.
>
> I'll check it ASAP, unless you do it before me.
>
> Thanks,
> Paolo
>
> [1] https://lkml.org/lkml/2020/1/31/94
>
>> Il giorno 2 lug 2020, alle ore 12:57, Dmitry Monakhov <dmonakhov@gmail.com> ha scritto:
>> 
>> commit db37a34c563b ("block, bfq: get a ref to a group when adding it to a service tree")
>> introduce leak forbfq_group and blkcg_gq objects because of get/put
>> imbalance. See trace balow:
>> -> blkg_alloc
>>   -> bfq_pq_alloc
>>     -> bfqg_get (+1)
>> ->bfq_activate_bfqq
>>  ->bfq_activate_requeue_entity
>>    -> __bfq_activate_entity
>>       ->bfq_get_entity
->>         ->bfqg_and_blkg_get (+1)  <==== : Note1
>> ->bfq_del_bfqq_busy
>>  ->bfq_deactivate_entity+0x53/0xc0 [bfq]
>>    ->__bfq_deactivate_entity+0x1b8/0x210 [bfq]
>>      -> bfq_forget_entity(is_in_service = true)
>> 	 entity->on_st_or_in_serv = false   <=== :Note2
>> 	 if (is_in_service)
>> 	     return;  ==> do not touch reference
>> -> blkcg_css_offline
>> -> blkcg_destroy_blkgs
>>  -> blkg_destroy
>>   -> bfq_pd_offline
>>    -> __bfq_deactivate_entity
>>         if (!entity->on_st_or_in_serv) /* true, because (Note2)
>> 		return false;
>> -> bfq_pd_free
>>    -> bfqg_put() (-1, byt bfqg->ref == 2) because of (Note2)
>> So bfq_group and blkcg_gq  will leak forever, see test-case below.
>> If fact bfq_group objects reference counting are quite different
>> from bfq_queue. bfq_groups object are referenced by blkcg_gq via
>> blkg_policy_data pointer, so  neither nor blkg_get() neither bfqg_get
>> required here.
>> 
>> 
>> This patch drop commit db37a34c563b ("block, bfq: get a ref to a group when adding it to a service tree")
>> and add corresponding comment.
>> 
>> ##TESTCASE_BEGIN:
>> #!/bin/bash
>> 
>> max_iters=${1:-100}
>> #prep cgroup mounts
>> mount -t tmpfs cgroup_root /sys/fs/cgroup
>> mkdir /sys/fs/cgroup/blkio
>> mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
>> 
>> # Prepare blkdev
>> grep blkio /proc/cgroups
>> truncate -s 1M img
>> losetup /dev/loop0 img
>> echo bfq > /sys/block/loop0/queue/scheduler
>> 
>> grep blkio /proc/cgroups
>> for ((i=0;i<max_iters;i++))
>> do
>>    mkdir -p /sys/fs/cgroup/blkio/a
>>    echo 0 > /sys/fs/cgroup/blkio/a/cgroup.procs
>>    dd if=/dev/loop0 bs=4k count=1 of=/dev/null iflag=direct 2> /dev/null
>>    echo 0 > /sys/fs/cgroup/blkio/cgroup.procs
>>    rmdir /sys/fs/cgroup/blkio/a
>>    grep blkio /proc/cgroups
>> done
>> ##TESTCASE_END:
>> 
>> Signed-off-by: Dmitry Monakhov <dmonakhov@gmail.com>
>> ---
>> block/bfq-cgroup.c  |  2 +-
>> block/bfq-iosched.h |  1 -
>> block/bfq-wf2q.c    | 15 +++++----------
>> 3 files changed, 6 insertions(+), 12 deletions(-)
>> 
>> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
>> index 68882b9..b791e20 100644
>> --- a/block/bfq-cgroup.c
>> +++ b/block/bfq-cgroup.c
>> @@ -332,7 +332,7 @@ static void bfqg_put(struct bfq_group *bfqg)
>> 		kfree(bfqg);
>> }
>> 
>> -void bfqg_and_blkg_get(struct bfq_group *bfqg)
>> +static void bfqg_and_blkg_get(struct bfq_group *bfqg)
>> {
>> 	/* see comments in bfq_bic_update_cgroup for why refcounting bfqg */
>> 	bfqg_get(bfqg);
>> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
>> index cd224aa..7038952 100644
>> --- a/block/bfq-iosched.h
>> +++ b/block/bfq-iosched.h
>> @@ -986,7 +986,6 @@ struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
>> struct blkcg_gq *bfqg_to_blkg(struct bfq_group *bfqg);
>> struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
>> struct bfq_group *bfq_create_group_hierarchy(struct bfq_data *bfqd, int node);
>> -void bfqg_and_blkg_get(struct bfq_group *bfqg);
>> void bfqg_and_blkg_put(struct bfq_group *bfqg);
>> 
>> #ifdef CONFIG_BFQ_GROUP_IOSCHED
>> diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
>> index 34ad095..6a363bb 100644
>> --- a/block/bfq-wf2q.c
>> +++ b/block/bfq-wf2q.c
>> @@ -529,13 +529,14 @@ static void bfq_get_entity(struct bfq_entity *entity)
>> {
>> 	struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity);
>> 
>> +	/* Grab reference only for bfq_queue's objects, bfq_group ones
>> +	 * are owned by blkcg_gq
>> +	 */
>> 	if (bfqq) {
>> 		bfqq->ref++;
>> 		bfq_log_bfqq(bfqq->bfqd, bfqq, "get_entity: %p %d",
>> 			     bfqq, bfqq->ref);
>> -	} else
>> -		bfqg_and_blkg_get(container_of(entity, struct bfq_group,
>> -					       entity));
>> +	}
>> }
>> 
>> /**
>> @@ -649,14 +650,8 @@ static void bfq_forget_entity(struct bfq_service_tree *st,
>> 
>> 	entity->on_st_or_in_serv = false;
>> 	st->wsum -= entity->weight;
>> -	if (is_in_service)
>> -		return;
>> -
>> -	if (bfqq)
>> +	if (bfqq && !is_in_service)
>> 		bfq_put_queue(bfqq);
>> -	else
>> -		bfqg_and_blkg_put(container_of(entity, struct bfq_group,
>> -					       entity));
>> }
>> 
>> /**
>> -- 
>> 2.7.4
>>