Re: [PATCH] memory-hotplug: fix a drain pcp bug when offline pages

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Hi Jiang,

On Wed, Aug 22, 2012 at 04:30:09PM +0800, Jiang Liu wrote:
> On 2012-8-22 16:14, Minchan Kim wrote:
> > On Wed, Aug 22, 2012 at 03:57:45PM +0800, qiuxishi wrote:
> >> On 2012-8-22 11:34, Minchan Kim wrote:
> >>> Hello Xishi,
> >>>
> >>> On Tue, Aug 21, 2012 at 08:12:05PM +0800, qiuxishi wrote:
> >>>> From: Xishi Qiu <qiuxishi@xxxxxxxxxx>
> >>>>
> >>>> When offline a section, we move all the free pages and pcp into MIGRATE_ISOLATE list first.
> >>>> start_isolate_page_range()
> >>>> 	set_migratetype_isolate()
> >>>> 		drain_all_pages(),
> >>>>
> >>>> Here is a problem, 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. So when finish migrating
> >>>> pages, the free pages from pcp may be allocated again, and faild in check_pages_isolated().
> >>>> drain_all_pages()
> >>>> 	drain_local_pages()
> >>>> 		drain_pages()
> >>>> 			free_pcppages_bulk()
> >>>> 				__free_one_page(page, zone, 0, page_private(page));
> >>>>
> >>>> If we add move_freepages_block() after drain_all_pages(), it can not sure that all the pcp
> >>>> will be moved into MIGRATE_ISOLATE list when the system works on high load. The free pages
> >>>> which from pcp may immediately be allocated again.
> >>>>
> >>>> I think the similar bug described in
> >>>
> >>> Yes. I reported the problem a few month ago but it's not real bug in practice
> >>> but found by my eyes during looking the code so I wanted to confirm the problem.
> >>>
> >>> Do you find that problem in real practice? or just code review?
> >>>
> >>
> >> I use /sys/devices/system/memory/soft_offline_page to offline a lot of pages when the
> >> system works on high load, then I find some unknown zero refcount pages, such as
> >> get_any_page: 0x650422: unknown zero refcount page type 19400c00000000
> >> get_any_page: 0x650867: unknown zero refcount page type 19400c00000000
> >>
> >> soft_offline_page()
> >> 	get_any_page()
> >> 		set_migratetype_isolate()
> >> 			drain_all_pages()
> >>
> >> I think after drain_all_pages(), pcp are moved into MIGRATE_MOVABLE list which managed by
> >> buddy allocator, but they are allocated and becaome pcp again as the system works on high
> >> load. There will be no this problem by applying this patch.
> >>
> >>> 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 ?
> >>
> >> Yes, you are right, I will try to find another way to fix this problem.
> >> How about doing this work in set_migratetype_isolate(), find the pcp and change the value
> >> of private to get_pageblock_migratetype(page)?
> >>
> > 
> > Allocator doesn't have any lock when he allocates the page from pcp.
> > How could you prevent race between allocator and memory-hotplug
> > routine(ie, set_migratetype_isolate) without hurting hotpath?
> Hi Minchan,
> 	I have thought about using a jump label in the hot path, which won't cause big
> performance drop, but it seems a little dirty. What's your thoughts?

I don't know static_key_false internal well.

1. Is it implemented by all archs?
2. How is it work? It's almost zero on all archs?
3. Don't we really have any solution other than hacking the hotpath
   (ie, order-0 page allocation)?
4. Please see my solution on above URL. Does it has any problem?

> 	migrate_type = page_private(page);
> 	if (static_key_false(&memory_hotplug_inprogress))
> 		migrate_type = get_pageblock_migratetype(page);
> 	__free_one_page(page, zone, 0, migrate_type);
> 	Regards!
> 	Gerry
> --
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Kind regards,
Minchan Kim
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