Re: [PATCH 28/48] writeback: implement and use mapping_congested()

[Tejun Heo]

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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