linux-kernel - Re: [PATCH v2] blkcg: allocate struct blkcg

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Date:   Fri, 3 Mar 2017 11:23:25 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Tahsin Erdogan <tahsin@...gle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        David Rientjes <rientjes@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] blkcg: allocate struct blkcg_gq outside request queue
 spinlock

Hello, Tahsin.

On Thu, Mar 02, 2017 at 02:33:11PM -0800, Tahsin Erdogan wrote:
> > And let blkg_create() verify these conditions after releasing and
> > regrabbing the lock.
> >
> > This also means that the init path can simply pass in GFP_KERNEL.
> 
> I tried that approach, but I encountered two issues that complicate things:
> 
> 1) Pushing down blk_queue_bypass(q) check in blkg_create() doesn't
> quite work because when blkcg_init_queue() calls blkg_create(), the
> queue is still in bypassing mode.
>
> 2) Pushing down blkcg_policy_enabled() doesn't work well either,
> because blkcg_init_queue() doesn't have a policy to pass down. We
> could let it pass a NULL parameter but that would make blkg_create
> more ugly.

I see.  It kinda really bothers me that we'll have two different modes
for non-atomic allocations.  Can't we bind both to the policy
parameter?  Skip the checks if policy is NULL?

Thanks.

-- 
tejun