Re: [PATCH 1/7] memcg : comment, style fixes for recent patch of move_parent

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 24 Jan 2011 11:14:02 +0100
Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Fri, Jan 21, 2011 at 03:37:26PM +0900, KAMEZAWA Hiroyuki wrote:
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > 
> > A fix for 987eba66e0e6aa654d60881a14731a353ee0acb4
> > 
> > A clean up for mem_cgroup_move_parent(). 
> >  - remove unnecessary initialization of local variable.
> >  - rename charge_size -> page_size
> >  - remove unnecessary (wrong) comment.
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > ---
> >  mm/memcontrol.c |   17 +++++++++--------
> >  1 file changed, 9 insertions(+), 8 deletions(-)
> > 
> > Index: mmotm-0107/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0107.orig/mm/memcontrol.c
> > +++ mmotm-0107/mm/memcontrol.c
> > @@ -2265,7 +2265,7 @@ static int mem_cgroup_move_parent(struct
> >  	struct cgroup *cg = child->css.cgroup;
> >  	struct cgroup *pcg = cg->parent;
> >  	struct mem_cgroup *parent;
> > -	int charge = PAGE_SIZE;
> > +	int page_size;
> >  	unsigned long flags;
> >  	int ret;
> >  
> > @@ -2278,22 +2278,23 @@ static int mem_cgroup_move_parent(struct
> >  		goto out;
> >  	if (isolate_lru_page(page))
> >  		goto put;
> > -	/* The page is isolated from LRU and we have no race with splitting */
> > -	charge = PAGE_SIZE << compound_order(page);
> > +
> > +	page_size = PAGE_SIZE << compound_order(page);
> 
> Okay, so you remove the wrong comment, but that does not make the code
> right.  What protects compound_order from reading garbage because the
> page is currently splitting?
> 

==
static int mem_cgroup_move_account(struct page_cgroup *pc,
                struct mem_cgroup *from, struct mem_cgroup *to,
                bool uncharge, int charge_size)
{
        int ret = -EINVAL;
        unsigned long flags;

        if ((charge_size > PAGE_SIZE) && !PageTransHuge(pc->page))
                return -EBUSY;
==

This is called under compound_lock(). Then, if someone breaks THP,
-EBUSY and retry.


Thanks,
-Kame