Re: [PATCH] bfq: fix blkio cgroup leakage

[Dmitry Monakhov <dmonakhov@gmail.com>]

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

>> Il giorno 8 lug 2020, alle ore 19:48, Dmitry Monakhov <dmonakhov@gmail.com> ha scritto:
>> 
>> 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.
>> 
>
> You are right, but incomplete.  No extra ref is needed for an entity
> that represents a bfq_queue.  And this consideration mistook me before
> I realized that that commit was needed.  The problem is that an entity
> may also represent a group of entities.  In that case no reference is
> taken through any bfq_queue.  The commit you want to remove takes this
> missing reference.
Sorry, It looks like I've mistyped sentance above, I ment to say bfq_group.
So here is my statement corrected:
 #IF CONFIG_BFQ_GROUP_IOSCHED is enabled:
 bfqd->root_group is holded by bfqd from bfq_init_queue()
 other *bfq_group* objects are owned by corresponding blkcg, reference get from bfq_pd_alloc()
 So *bfq_group* can not disappear under us.

So no extra reference is required for entity represents bfq_group. Commit is not required.
>
> Paolo
>
>> #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
>>>>