Re: [PATCH] mm: compaction: Abort compaction if too many pages are isolated and caller is asynchronous

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, Jun 03, 2011 at 05:21:56AM +0900, Minchan Kim wrote:
> On Thu, Jun 02, 2011 at 08:23:02PM +0200, Andrea Arcangeli wrote:
> > On Tue, May 31, 2011 at 11:38:30PM +0900, Minchan Kim wrote:
> > > > Yes. You find a new BUG.
> > > > It seems to be related to this problem but it should be solved although
> > > 
> > >  typo : It doesn't seem to be.
> > 
> > This should fix it, but I doubt it matters for this problem.
> > 
> > ===
> > Subject: mm: no page_count without a page pin
> > 
> > From: Andrea Arcangeli <aarcange@redhat.com>
> > 
> > It's unsafe to run page_count during the physical pfn scan.
> > 
> > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> > ---
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index faa0a08..e41e78a 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1124,8 +1124,18 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> >  					nr_lumpy_dirty++;
> >  				scan++;
> >  			} else {
> > -				/* the page is freed already. */
> > -				if (!page_count(cursor_page))
> > +				/*
> > +				 * We can't use page_count() as that
> > +				 * requires compound_head and we don't
> > +				 * have a pin on the page here. If a
> > +				 * page is tail, we may or may not
> > +				 * have isolated the head, so assume
> > +				 * it's not free, it'd be tricky to
> 
> Isn't it rather aggressive?
> I think cursor page is likely to be PageTail rather than PageHead.
> Could we handle it simply with below code?
> 
> get_page(cursor_page)
> /* The page is freed already */
> if (1 == page_count(cursor_page)) {
> 	put_page(cursor_page)
> 	continue;
> }
> put_page(cursor_page);
> 

Now that I look code more, it would meet VM_BUG_ON of get_page if the page is really
freed. I think if we hold zone->lock to prevent prep_new_page racing, it would be okay.
But it's rather overkill so I will add my sign to your patch if we don't have better idea
until tomorrow. :)

-- 
Kind regards
Minchan Kim

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