Date:	Mon, 03 Feb 2014 10:16:50 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Minchan Kim <minchan@...nel.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Wei Yongjun <yongjun_wei@...ndmicro.com.cn>,
	Tang Chen <tangchen@...fujitsu.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] improve robustness on handling migratetype

On 02/03/2014 08:45 AM, Joonsoo Kim wrote:
> On Wed, Jan 29, 2014 at 05:52:41PM +0100, Vlastimil Babka wrote:
>> On 01/10/2014 09:48 AM, Joonsoo Kim wrote:
>>> On Thu, Jan 09, 2014 at 09:27:20AM +0000, Mel Gorman wrote:
>>>> On Thu, Jan 09, 2014 at 04:04:40PM +0900, Joonsoo Kim wrote:
>>>>> Hello,
>>>>>
>>>>> I found some weaknesses on handling migratetype during code review and
>>>>> testing CMA.
>>>>>
>>>>> First, we don't have any synchronization method on get/set pageblock
>>>>> migratetype. When we change migratetype, we hold the zone lock. So
>>>>> writer-writer race doesn't exist. But while someone changes migratetype,
>>>>> others can get migratetype. This may introduce totally unintended value
>>>>> as migratetype. Although I haven't heard of any problem report about
>>>>> that, it is better to protect properly.
>>>>>
>>>>
>>>> This is deliberate. The migratetypes for the majority of users are advisory
>>>> and aimed for fragmentation avoidance. It was important that the cost of
>>>> that be kept as low as possible and the general case is that migration types
>>>> change very rarely. In many cases, the zone lock is held. In other cases,
>>>> such as splitting free pages, the cost is simply not justified.
>>>>
>>>> I doubt there is any amount of data you could add in support that would
>>>> justify hammering the free fast paths (which call get_pageblock_type).
>>>
>>> Hello, Mel.
>>>
>>> There is a possibility that we can get unintended value such as 6 as migratetype
>>> if reader-writer (get/set pageblock_migratetype) race happends. It can be
>>> possible, because we read the value without any synchronization method. And
>>> this migratetype, 6, has no place in buddy freelist, so array index overrun can
>>> be possible and the system can break, although I haven't heard that it occurs.
>>
>> Hello,
>>
>> it seems this can indeed happen. I'm working on memory compaction
>> improvements and in a prototype patch, I'm basically adding calls of
>> start_isolate_page_range() undo_isolate_page_range() some functions
>> under compact_zone(). With this I've seen occurrences of NULL
>> pointers in move_freepages(), free_one_page() in places where
>> free_list[migratetype] is manipulated by e.g. list_move(). That lead
>> me to question the value of migratetype and I found this thread.
>> Adding some debugging in get_pageblock_migratetype() and voila, I
>> get a value of 6 being read.
>>
>> So is it just my patch adding a dangerous situation, or does it exist in
>> mainline as well? By looking at free_one_page(), it uses zone->lock, but
>> get_pageblock_migratetype() is called by its callers
>> (free_hot_cold_page() or __free_pages_ok()) outside of the lock.
>> This determined migratetype is then used under free_one_page() to
>> access a free_list.
>>
>> It seems that this could race with set_pageblock_migratetype()
>> called from try_to_steal_freepages() (despite the latter being
>> properly locked). There are also other callers but those seem to be
>> either limited to initialization and isolation, which should be rare
>> (?).
>> However, try_to_steal_freepages can occur repeatedly.
>> So I assume that the race happens but never manifests as a fatal
>> error as long as MIGRATE_UNMOVABLE, MIGRATE_RECLAIMABLE and
>> MIGRATE_MOVABLE
>> values are used. Only MIGRATE_CMA and MIGRATE_ISOLATE have values
>> with bit 4 enabled and can thus result in invalid values due to
>> non-atomic access.
>>
>> Does that make sense to you and should we thus proceed with patching
>> this race?
>>
>
> Hello,
>
> This race is possible without your prototype patch, however, on very low
> probability. Some codes related to memory failure use set_migratetype_isolate()
> which could result in this race.
>
> Although it may be very rare case and not critical, it is better to fix
> this race. I prefer that we don't depend on luck. :)

I agree :) I also don't like the possibility that the non-fatal type of 
race (where higher-order bits are not involved) occurs and can hurt 
anti-fragmentation, or even suddenly become a problem in the future if 
e.g. more migratetypes are added. I'll try to quantify that with a debug 
patch.

> Mel's suggestion looks good to me. Do you have another idea?

No, it sounds good so I'm going to work on this as outlined.

> Thanks.
>

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