On Mon, Jan 24, 2011 at 07:15:35PM +0900, KAMEZAWA Hiroyuki wrote:
> On Mon, 24 Jan 2011 11:14:02 +0100
> Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> > On Fri, Jan 21, 2011 at 03:37:26PM +0900, KAMEZAWA Hiroyuki wrote:
> > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxxxx>
> > > ---
> > > 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.
This charge_size contains exactly the garbage you just read from an
unprotected compound_order(). It could be anything if the page is
split concurrently.
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