Re: [PATCH 3/3] memory_hotplug: Don't modify the zone_start_pfn outside of zone_span_writelock()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Hi, KOSAKI

On 09/28/2012 06:30 AM, KOSAKI Motohiro wrote:
> (9/27/12 2:47 AM), Lai Jiangshan wrote:
>> The __add_zone() maybe call sleep-able init_currently_empty_zone()
>> to init wait_table,
> 
> This doesn't explain why sleepable is critical important. I think sleepable
> is jsut unrelated. The fact is only: to write zone->zone_start_pfn require
> zone_span_writelock, but init_currently_empty_zone() doesn't take it.

You are right, sleepable is not critical important, but the lock is critical.

I am Sorry that I added "sleep-able" and misled guys.

Actually I want to say:

1) to write zone->zone_start_pfn require zone_span_writelock
2) init_currently_empty_zone() is sleepable, so we can't use zone_span_writelock()
   protect the whole init_currently_empty_zone().
3) so we have to move the modification code out of init_currently_empty_zone()
   as this patch does.

> 
> 
>>
>> But this function also modifies the zone_start_pfn without any lock.
>> It is bugy.
> 
> buggy?
> 
> 
>> So we move this modification out, and we ensure the modification
>> of zone_start_pfn is only done with zone_span_writelock() held or in booting.
>>
>> Since zone_start_pfn is not modified by init_currently_empty_zone()
>> grow_zone_span() needs to check zone_start_pfn before update it.
>>
>> CC: Mel Gorman <mel@csn.ul.ie>
>> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
>> Reported-by: Yasuaki ISIMATU <isimatu.yasuaki@jp.fujitsu.com>
>> Tested-by: Wen Congyang <wency@cn.fujitsu.com>
>> ---
>>  mm/memory_hotplug.c |    2 +-
>>  mm/page_alloc.c     |    3 +--
>>  2 files changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index b62d429b..790561f 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -205,7 +205,7 @@ static void grow_zone_span(struct zone *zone, unsigned long start_pfn,
>>  	zone_span_writelock(zone);
>>  
>>  	old_zone_end_pfn = zone->zone_start_pfn + zone->spanned_pages;
>> -	if (start_pfn < zone->zone_start_pfn)
>> +	if (!zone->zone_start_pfn || start_pfn < zone->zone_start_pfn)
>>  		zone->zone_start_pfn = start_pfn;
> 
> Wrong. zone->zone_start_pfn==0 may be valid pfn. You shouldn't assume it is uninitialized
> value.

Good catch, I will use zone->spanned_pages instead.


Thanks,
Lai

> 
> 
>>  
>>  	zone->spanned_pages = max(old_zone_end_pfn, end_pfn) -
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index c13ea75..2545013 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3997,8 +3997,6 @@ int __meminit init_currently_empty_zone(struct zone *zone,
>>  		return ret;
>>  	pgdat->nr_zones = zone_idx(zone) + 1;
>>  
>> -	zone->zone_start_pfn = zone_start_pfn;
>> -
>>  	mminit_dprintk(MMINIT_TRACE, "memmap_init",
>>  			"Initialising map node %d zone %lu pfns %lu -> %lu\n",
>>  			pgdat->node_id,
>> @@ -4465,6 +4463,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
>>  		ret = init_currently_empty_zone(zone, zone_start_pfn,
>>  						size, MEMMAP_EARLY);
>>  		BUG_ON(ret);
>> +		zone->zone_start_pfn = zone_start_pfn;
>>  		memmap_init(size, nid, j, zone_start_pfn);
>>  		zone_start_pfn += size;
>>  	}
>>
> 
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