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

[Minchan Kim]

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?

> 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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-- 
Kind regards,
Minchan Kim
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