Re: [PATCH v2 3/3] memory-hotplug: bug fix race between isolationand allocation

[Yasuaki Ishimatsu]

Hi, Minchan,

2012/09/06 16:30, Minchan Kim wrote:
> Hello Yasuaki,
>
> On Thu, Sep 06, 2012 at 04:17:54PM +0900, Yasuaki Ishimatsu wrote:
> > Hi Minchan,
> >
> > 2012/09/06 14:16, Minchan Kim 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));
> > >
> > > This patch checks page's migratetype in freelist in __test_page_isolated_in_pageblock.
> > > So now __test_page_isolated_in_pageblock can check the page caused by above race and
> > > can fail of memory offlining.
> > >
> > > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > > ---
> > >    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 87a7929..7ba7405 100644
> > > --- a/mm/page_isolation.c
> > > +++ b/mm/page_isolation.c
> > > @@ -193,8 +193,11 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn)
> > >    			continue;
> > >    		}
> > >    		page = pfn_to_page(pfn);
> > > -		if (PageBuddy(page))
> > > +		if (PageBuddy(page)) {
> > > +			if (get_freepage_migratetype(page) != MIGRATE_ISOLATE)
> > > +				break;
> > >    			pfn += 1 << page_order(page);
> > > +		}
> >
> > >    		else if (page_count(page) == 0 &&
> > >    			get_freepage_migratetype(page) == MIGRATE_ISOLATE)
> >
> > When do the if statement, the page may be used by someone.
>
> I can't understand your point.
> We already hold zone->lock so that allocator and this function should be atomic
> when the page is in free_list.
> If I miss something, could you elaborate it more?

According to your description, the page might be allocated by someone
at this point. So some value might be set the page->index by the
intended purpose. Thus page->index has the potential to become
MIGRATE_ISOLATE value.

Thanks,
Yasuaki Ishimatsu   

> > In this case, page->index may have some number. If the number is same as
> > MIGRATE_ISOLATE, the code goes worng.
> >
> > Thanks,
> > Yasuaki Ishimatsu
> >
> > >    			pfn += 1;
> >
> >
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