Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2023/01/10 2:23, Tejun Heo 写道:
> Yeah, that's unfortunate. There are several options here:
> 
> 1. Do what you originally suggested - bypass to root after offline. I feel
>     uneasy about this. Both iolatency and throtl clear their configs on
>     offline but that's punting to the parent. For iocost it'd be bypassing
>     all controls, which can actually be exploited.
> 
> 2. Make all possible IO issuers use blkcg_[un]pin_online() and shift the
>     iocost shutdown to pd_offline_fn(). This likely is the most canonical
>     solution given the current situation but it's kinda nasty to add another
>     layer of refcnting all over the place.
> 
> 3. Order blkg free so that parents are never freed before children. You did
>     this by adding refcnts in iocost but shouldn't it be possible to simply
>     shift blkg_put(blkg->parent) in __blkg_release() to blkg_free_workfn()?

As I tried to explain before, we can make sure blkg_free() is called
in order, but blkg_free() from remove cgroup can concurrent with
deactivate policy, and we can't guarantee the order of ioc_pd_free()
that is called both from blkg_free() and blkcg_deactivate_policy().
Hence I don't think #3 is possible.

I personaly prefer #1, I don't see any real use case about the defect
that you described, and actually in cgroup v1 blk-throtl is bypassed to
no limit as well.

I'm not sure about #2, that sounds a possible solution but I'm not quite
familiar with the implementations here.

Consider that bfq already has such refcounting for bfqg, perhaps
similiar refcounting is acceptable?

Thanks,
Kuai