Re: [RFC 3/3] memory-hotplug: bug fix race between isolation andallocation

[Minchan Kim]

Hi Bob,

On Tue, Jul 17, 2012 at 06:13:17PM +0800, Bob Liu wrote:
> Hi Minchan,
> 
> On Tue, Jul 17, 2012 at 3:01 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > Like below, memory-hotplug makes race between page-isolation
> > and page-allocation so it can hit BUG_ON in __offline_isolated_pages.
> >
> >         CPU A                                   CPU B
> >
> > start_isolate_page_range
> > set_migratetype_isolate
> > spin_lock_irqsave(zone->lock)
> >
> >                                 free_hot_cold_page(Page A)
> >                                 /* without zone->lock */
> >                                 migratetype = get_pageblock_migratetype(Page A);
> >                                 /*
> >                                  * Page could be moved into MIGRATE_MOVABLE
> >                                  * of per_cpu_pages
> >                                  */
> >                                 list_add_tail(&page->lru, &pcp->lists[migratetype]);
> >
> > set_pageblock_isolate
> > move_freepages_block
> > drain_all_pages
> >
> >                                 /* Page A could be in MIGRATE_MOVABLE of free_list. */
> >
> > check_pages_isolated
> > __test_page_isolated_in_pageblock
> > /*
> >  * We can't catch freed page which
> >  * is free_list[MIGRATE_MOVABLE]
> >  */
> > if (PageBuddy(page A))
> >         pfn += 1 << page_order(page A);
> >
> >                                 /* So, Page A could be allocated */
> >
> > __offline_isolated_pages
> > /*
> >  * BUG_ON hit or offline page
> >  * which is used by someone
> >  */
> > BUG_ON(!PageBuddy(page A));
> >
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> > I found this problem during code review so please confirm it.
> > Kame?
> >
> >  mm/page_isolation.c |    5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index acf65a7..4699d1f 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -196,8 +196,11 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn)
> >                         continue;
> >                 }
> >                 page = pfn_to_page(pfn);
> > -               if (PageBuddy(page))
> > +               if (PageBuddy(page)) {
> >                         pfn += 1 << page_order(page);
> > +                       if (get_page_migratetype(page) != MIGRATE_ISOLATE)
> > +                               break;
> > +               }
> 
> test_page_isolated() already have check
> get_pageblock_migratetype(page) != MIGRATE_ISOLATE.
> 

That's why I send a patch.
As I describe in description, pageblock migration type of get_page_migratetype(page)
is inconsistent with free_list[migrationtype].
I mean get_pageblock_migratetype(page) will return MIGRATE_ISOLATE but the page would be
in free_list[MIGRATE_MOVABLE] so it could be allocated for someone if that race happens.

-- 
Kind regards,
Minchan Kim
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