linux-kernel - Re: [PATCH 4/5] mm/swapfile: refcount block and queue before using blkcg_schedule

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Message-ID: <20200416061048.GA1342@infradead.org>
Date:   Wed, 15 Apr 2020 23:10:48 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Alan Jenkins <alan.christopher.jenkins@...il.com>,
        axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 4/5] mm/swapfile: refcount block and queue before using
 blkcg_schedule_throttle()

On Wed, Apr 15, 2020 at 01:19:15PM +0000, Luis Chamberlain wrote:
> >  	if (current->throttle_queue)
> >  		return;
> > +	if (unlikely(current->flags & PF_KTHREAD))
> > +		return;
> >  
> >  	spin_lock(&swap_avail_lock);
> >  	plist_for_each_entry_safe(si, next, &swap_avail_heads[node],
> >  				  avail_lists[node]) {
> > -		if (si->bdev) {
> > -			blkcg_schedule_throttle(bdev_get_queue(si->bdev),
> > -						true);
> > -			break;
> > +		if (!si->bdev)
> > +			continue;
> > +		if (blk_get_queue(dev_get_queue(si->bdev))) {
> > +			current->throttle_queue = dev_get_queue(si->bdev);
> > +			current->use_memdelay = true;
> > +			set_notify_resume(current);
> >  		}
> > +		break;
> >  	}
> >  	spin_unlock(&swap_avail_lock);
> >  }
> 
> Sorry, its not clear to me  who calls the respective blk_put_queue()
> here?

If you look at blkcg_schedule_throttle, it only puts the queue that
was in current->throttle_queue.  But mem_cgroup_throttle_swaprate
exits early when current->throttle_queue is non-zero (first two lines
quote above).  So when called from mem_cgroup_throttle_swaprate,
blkcg_schedule_throttle should never actually put a queue.  Open
coding the few relevant lines from blkcg_schedule_throttle in
mem_cgroup_throttle_swaprate makes that obvious.