Re: [PATCH v2 3/3] blkcg: implement sync() isolation

[Andrea Righi <andrea.righi@canonical.com>]

On Thu, Mar 07, 2019 at 05:07:01PM -0500, Josef Bacik wrote:
> On Thu, Mar 07, 2019 at 07:08:34PM +0100, Andrea Righi wrote:
> > Keep track of the inodes that have been dirtied by each blkcg cgroup and
> > make sure that a blkcg issuing a sync() can trigger the writeback + wait
> > of only those pages that belong to the cgroup itself.
> > 
> > This behavior is applied only when io.sync_isolation is enabled in the
> > cgroup, otherwise the old behavior is applied: sync() triggers the
> > writeback of any dirty page.
> > 
> > Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> > ---
> >  block/blk-cgroup.c         | 47 ++++++++++++++++++++++++++++++++++
> >  fs/fs-writeback.c          | 52 +++++++++++++++++++++++++++++++++++---
> >  fs/inode.c                 |  1 +
> >  include/linux/blk-cgroup.h | 22 ++++++++++++++++
> >  include/linux/fs.h         |  4 +++
> >  mm/page-writeback.c        |  1 +
> >  6 files changed, 124 insertions(+), 3 deletions(-)
> > 
> > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> > index 4305e78d1bb2..7d3b26ba4575 100644
> > --- a/block/blk-cgroup.c
> > +++ b/block/blk-cgroup.c
> > @@ -1480,6 +1480,53 @@ void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi)
> >  	spin_unlock(&blkcg_wb_sleeper_lock);
> >  	rcu_read_unlock();
> >  }
> > +
> > +/**
> > + * blkcg_set_mapping_dirty - set owner of a dirty mapping
> > + * @mapping: target address space
> > + *
> > + * Set the current blkcg as the owner of the address space @mapping (the first
> > + * blkcg that dirties @mapping becomes the owner).
> > + */
> > +void blkcg_set_mapping_dirty(struct address_space *mapping)
> > +{
> > +	struct blkcg *curr_blkcg, *blkcg;
> > +
> > +	if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK) ||
> > +	    mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
> > +		return;
> > +
> > +	rcu_read_lock();
> > +	curr_blkcg = blkcg_from_current();
> > +	blkcg = blkcg_from_mapping(mapping);
> > +	if (curr_blkcg != blkcg) {
> > +		if (blkcg)
> > +			css_put(&blkcg->css);
> > +		css_get(&curr_blkcg->css);
> > +		rcu_assign_pointer(mapping->i_blkcg, curr_blkcg);
> > +	}
> > +	rcu_read_unlock();
> > +}
> > +
> > +/**
> > + * blkcg_set_mapping_clean - clear the owner of a dirty mapping
> > + * @mapping: target address space
> > + *
> > + * Unset the owner of @mapping when it becomes clean.
> > + */
> > +
> > +void blkcg_set_mapping_clean(struct address_space *mapping)
> > +{
> > +	struct blkcg *blkcg;
> > +
> > +	rcu_read_lock();
> > +	blkcg = rcu_dereference(mapping->i_blkcg);
> > +	if (blkcg) {
> > +		css_put(&blkcg->css);
> > +		RCU_INIT_POINTER(mapping->i_blkcg, NULL);
> > +	}
> > +	rcu_read_unlock();
> > +}
> >  #endif
> >  
> 
> Why do we need this?  We already have the inode_attach_wb(), which has the
> blkcg_css embedded in it for whoever dirtied the inode first.  Can we not just
> use that?  Thanks,
> 
> Josef

I'm realizing only now that inode_attach_wb() also has blkcg embedded
in addition to the memcg. I think I can use that and drop these
blkcg_set_mapping_dirty/clean()..

Thanks,
-Andrea