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Date: Fri, 27 Mar 2015 17:46:13 -0400
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 28/48] writeback: implement and use mapping_congested()
Message-ID: <20150327214613.GF638@htj.duckdns.org>
References: <1427086499-15657-1-git-send-email-tj@kernel.org>
 <1427086499-15657-29-git-send-email-tj@kernel.org>
 <20150327180626.GA19117@redhat.com>
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To: Vivek Goyal <vgoyal@redhat.com>
Cc: axboe@kernel.dk, linux-kernel@vger.kernel.org, jack@suse.cz, hch@infradead.org, hannes@cmpxchg.org, linux-fsdevel@vger.kernel.org, lizefan@huawei.com, cgroups@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.cz, clm@fb.com, fengguang.wu@intel.com, david@fromorbit.com, gthelen@google.com

Hello, Vivek.

On Fri, Mar 27, 2015 at 02:06:26PM -0400, Vivek Goyal wrote:
...
> > +int mapping_congested(struct address_space *mapping,
> > +		      struct task_struct *task, int cong_bits)
> > +{
> > +	struct inode *inode = mapping->host;
> > +	struct backing_dev_info *bdi = inode_to_bdi(inode);
> > +	struct bdi_writeback *wb;
> > +	int ret = 0;
> > +
> > +	if (!inode || !inode_cgwb_enabled(inode))
> > +		return wb_congested(&bdi->wb, cong_bits);
> > +
> > +	rcu_read_lock();
> > +	wb = wb_find_current(bdi);
> 
> I am wondering that why do we lookup bdi_writeback using blkcg of
> task and why not use the bdi_writeback associated with inode?
> 
> IIUC, whole idea is to attach an inode to bdi_writeback (and
> change it later if need be) and that writeback is used for
> controlling IO to that inode. And blkcg associated with the
> writeback will be put in bio which in turn will be used
> by block layer.
> 
> IOW, blkcg of a bio gets decided by the bdi_writeback
> attached to inode and current writer does not seem to
> matter. So I am not sure why mapping_congested() should
> take task's blkcg into consideration instead of just
> taking bdi_writeback from inode and see if it is congested
> or not.

Yeap, I agree that attributing to the inode's blkcg makes more sense.
I need to think more about it but will prolly change it to use
inode->i_wb instead.

Thanks.

-- 
tejun

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