Re: [PATCH] memcg: fix mapcount check in move charge code foranonymous page

[Hugh Dickins]

On Mon, 5 Mar 2012, Daisuke Nishimura wrote:
> Hi, Horiguchi-san.
> On Fri,  2 Mar 2012 15:35:08 -0500
> Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> wrote:
> 
> > Currently charge on shared anonyous pages is supposed not to moved
> > in task migration. To implement this, we need to check that mapcount > 1,
> > instread of > 2. So this patch fixes it.
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > ---
> >  mm/memcontrol.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git linux-next-20120228.orig/mm/memcontrol.c linux-next-20120228/mm/memcontrol.c
> > index b6d1bab..785f6d3 100644
> > --- linux-next-20120228.orig/mm/memcontrol.c
> > +++ linux-next-20120228/mm/memcontrol.c
> > @@ -5102,7 +5102,7 @@ static struct page *mc_handle_present_pte(struct vm_area_struct *vma,
> >  		return NULL;
> >  	if (PageAnon(page)) {
> >  		/* we don't move shared anon */
> > -		if (!move_anon() || page_mapcount(page) > 2)
> > +		if (!move_anon() || page_mapcount(page) > 1)
> >  			return NULL;
> >  	} else if (!move_file())
> >  		/* we ignore mapcount for file pages */
> > -- 
> > 1.7.7.6
> > 
> Sorry, it's my fault..
> Thank you for catching this.
> 
> Reviewed-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

I'm perversely sorry to see this fix already wing its way into 3.3-rc,
but never mind.

I was puzzling over that same "> 2" test when thinking through the
stats move locking, and again when swap accounting appeared to be
broken through and through (now fixed by two-liner in page_cgroup.c).

Why is there any test on page_mapcount(page) there at all?
2.6.34 comments it
	* TODO: We don't move charges of shared(used by multiple
	* processes) pages for now.
as if it's an unwelcome restriction to be eliminated later.

I don't understand why it was ever there, and would like to remove
it (and update the Documentation file) - just to remove a little
unnecessary complication, including mem_cgroup_count_swap_user().

The file case moves account, even when the page is not mapped into
this address space, even when it's mapped into a thousand others.

Why treat the anonymous so differently here?  I'd have thought it
quite likely (by no means certain, but quite likely) that when you
move a task sharing an anon page from one cg to another, you'll
move the other task(s) sharing it immediately after - strange that
these shared pages should then get left behind.

I was pleased by the "> 2" bug, there almost all the life of
move_charge_at_immigrate, demonstrating that nobody was depending
upon the documented behaviour.

I've a few more cleanups in the swap accounting area, I guess I
should just post this change along with them and we discuss then,
unless you can enlighten me what it's about before I get there.

Thanks,
Hugh
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