Re: [PATCH 3/3] memory-hotplug: bug fix race between isolation and allocation

[Minchan Kim <minchan@kernel.org>]

Hello Xishi,

On Thu, Sep 06, 2012 at 10:35:39AM +0800, qiuxishi wrote:
> On 2012/9/5 17:40, Mel Gorman wrote:
> 
> > On Wed, Sep 05, 2012 at 04:26:02PM +0900, 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
> 
> I think here is the problem you want to fix, it is not sure that pcp will be moved
> into MIGRATE_ISOLATE list. They may be moved into MIGRATE_MOVABLE list because
> page_private() maybe 2, it uses page_private() not get_pageblock_migratetype()
> 
> So when finish migrating pages, the free pages from pcp may be allocated again, and
> failed in check_pages_isolated().
> 
> drain_all_pages()
> 	drain_local_pages()
> 		drain_pages()
> 			free_pcppages_bulk()
> 				__free_one_page(page, zone, 0, page_private(page))
> 
> I reported this problem too. http://marc.info/?l=linux-mm&m=134555113706068&w=2
> How about this change:
> 	free_pcppages_bulk()
> 		__free_one_page(page, zone, 0, get_pageblock_migratetype(page))

I already explained why it was not good solution.
Again, here it goes from my previous reply.

"
Anyway, I don't like your approach which I already considered because it hurts hotpath
while the race is really unlikely. Get_pageblock_migratetype is never trivial.
We should avoid the overhead in hotpath and move into memory-hotplug itself.
Do you see my patch in https://patchwork.kernel.org/patch/1225081/ ?
"

-- 
Kind regards,
Minchan Kim

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