Re: [PATCH 2/2] mem-hotplug: Fix wrong check for zone->pageset initialization in online_pages().

[Tang Chen]

On 11/04/2014 04:10 PM, Yasuaki Ishimatsu wrote:
> (2014/10/31 18:46), Tang Chen wrote:
>> When we are doing memory hot-add, the following functions are called:
>>
>> add_memory()
>> |--> hotadd_new_pgdat()
>>       |--> free_area_init_node()
>>            |--> free_area_init_core()
>>                 |--> zone->present_pages = realsize;           /* 1. zone is populated */
>>                      |--> zone_pcp_init()
>>                           |--> zone->pageset = &boot_pageset;  /* 2. zone->pageset is set to boot_pageset */
>>
>> There are two problems here:
>> 1. Zones could be populated before any memory is onlined.
>> 2. All the zones on a newly added node have the same pageset pointing to boot_pageset.
>>
>> The above two problems will result in the following problem:
>> When we online memory on one node, e.g node2, the following code is executed:
>>
>> online_pages()
>> {
>>          ......
>>          if (!populated_zone(zone)) {
>>                  need_zonelists_rebuild = 1;
>>                  build_all_zonelists(NULL, zone);
>>          }
>>          ......
>> }
>>
>> Because of problem 1, the zone has been populated, and the build_all_zonelists()
>>                        will never called. zone->pageset won't be updated.
>> Because of problem 2, All the zones on a newly added node have the same pageset
>>                        pointing to boot_pageset.
>> And as a result, when we online memory on node2, node3's meminfo will corrupt.
>> Pages on node2 may be freed to node3.
>>
>> # for ((i = 2048; i < 2064; i++)); do echo online_movable > /sys/devices/system/node/node2/memory$i/state; done
>> # cat /sys/devices/system/node/node2/meminfo
>> Node 2 MemTotal:       33554432 kB
>> Node 2 MemFree:        33549092 kB
>> Node 2 MemUsed:            5340 kB
>> ......
>> # cat /sys/devices/system/node/node3/meminfo
>> Node 3 MemTotal:              0 kB
>> Node 3 MemFree:               248 kB                    /* corrupted */
>> Node 3 MemUsed:               0 kB
>> ......
>>
>> We have to populate some zones before onlining memory, otherwise no memory could be onlined.
>> So when onlining pages, we should also check if zone->pageset is pointing to boot_pageset.
>>
>> Signed-off-by: Gu Zheng <guz.fnst@xxxxxxxxxxxxxx>
>> Signed-off-by: Tang Chen <tangchen@xxxxxxxxxxxxxx>
>> ---
>>   include/linux/mm.h  | 1 +
>>   mm/memory_hotplug.c | 6 +++++-
>>   mm/page_alloc.c     | 5 +++++
>>   3 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 02d11ee..83e6505 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1732,6 +1732,7 @@ void warn_alloc_failed(gfp_t gfp_mask, int order, const char *fmt, ...);
>>   
>>   extern void setup_per_cpu_pageset(void);
>>   
>> +extern bool zone_pcp_initialized(struct zone *zone);
>>   extern void zone_pcp_update(struct zone *zone);
>>   extern void zone_pcp_reset(struct zone *zone);
>>   
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 3ab01b2..bc0de0f 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1013,9 +1013,13 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
>>   	 * If this zone is not populated, then it is not in zonelist.
>>   	 * This means the page allocator ignores this zone.
>>   	 * So, zonelist must be updated after online.
>> +	 *
>> +	 * If this zone is populated, zone->pageset could be initialized
>> +	 * to boot_pageset for the first time a node is added. If so,
>> +	 * zone->pageset should be allocated.
>>   	 */
>>   	mutex_lock(&zonelists_mutex);
>> -	if (!populated_zone(zone)) {
>> +	if (!populated_zone(zone) || !zone_pcp_initialized(zone)) {
>>   		need_zonelists_rebuild = 1;
>>   		build_all_zonelists(NULL, zone);
>>   	}
> Why does zone->present_pages of the hot-added memroy have valid value?
> In my understading, the present_pages is incremented/decremented by memory
> online/offline. So when hot adding memory, the zone->present_pages of the
> memory should be 0.

Before zone->managed_pages was introduced, zone->present_pages had been
abused.
It had two meaning:
1. pages existing in the zone
2. pages managed by buddy system in the zone

So, zone->managed_pages was introduced. Now, it looks like:
1. zone->present_pages is the pages existing in the zone
2. zone->managed_pages is the pages managed by buddy system

So when hot adding memory, the zone->present_pages should not be 0.
zone->managed_pages should be 0. And here, since zone->managed_pages
is updated in online_pages_range() callback, I think we should remove
zone->present_pages and node_present_pages updates from online_pages().


Furthermore, since we can online pages or online_movable pages,
zones in a node could be changed. So we should update zone->present_pages
when pfn is moved left or right.

Thanks.

>
> Thanks,
> Yasuaki Ishimatsu
>
>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 736d8e1..4ff1540 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6456,6 +6456,11 @@ void __meminit zone_pcp_update(struct zone *zone)
>>   }
>>   #endif
>>   
>> +bool zone_pcp_initialized(struct zone *zone)
>> +{
>> +	return (zone->pageset != &boot_pageset);
>> +}
>> +
>>   void zone_pcp_reset(struct zone *zone)
>>   {
>>   	unsigned long flags;
>>
>
> .
>

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