Subject: Re: [PATCH 1/10] memory-hotplug : check whether memory is offline or not when removing memory

From: Wen Congyang <[email-masked]>
Date: Fri, 19 Oct 2012 22:15:56 +0800

At 2012/10/19 18:44, Wen Congyang Wrote:
> At 10/06/2012 03:27 AM, KOSAKI Motohiro Wrote:
>> On Thu, Oct 4, 2012 at 10:25 PM, Yasuaki Ishimatsu
>> <[email-masked]>  wrote:
>>> When calling remove_memory(), the memory should be offline. If the function
>>> is used to online memory, kernel panic may occur.
>>>
>>> So the patch checks whether memory is offline or not.
>>
>> You don't explain WHY we need the check.
>
> This patch is no necessary now, because the newest kernel has checked
> it.

I think it again, and found that this check is necessary. Because we only
lock memory hotplug when offlining pages. Here is the steps to offline and
remove memory:

1. lock memory hotplug
2. offline a memory section
3. unlock memory hotplug
4. repeat 1-3 to offline all memory sections
5. lock memory hotplug
6. remove memory
7. unlock memory hotplug

All memory sections must be offlined before removing memory. But we 
don't hold
the lock in the whole operation. So we should check whether all memory 
sections
are offlined before step6.

>
> Thanks
> Wen Congyang
>
>>
>>
>>> CC: David Rientjes<[email-masked]>
>>> CC: Jiang Liu<[email-masked]>
>>> CC: Len Brown<[email-masked]>
>>> CC: Christoph Lameter<[email-masked]>
>>> Cc: Minchan Kim<[email-masked]>
>>> CC: Andrew Morton<[email-masked]>
>>> CC: KOSAKI Motohiro<[email-masked]>
>>> Signed-off-by: Wen Congyang<[email-masked]>
>>> Signed-off-by: Yasuaki Ishimatsu<[email-masked]>
>>>
>>> ---
>>>   drivers/base/memory.c  |   39 +++++++++++++++++++++++++++++++++++++++
>>>   include/linux/memory.h |    5 +++++
>>>   mm/memory_hotplug.c    |   17 +++++++++++++++--
>>>   3 files changed, 59 insertions(+), 2 deletions(-)
>>>
>>> Index: linux-3.6/drivers/base/memory.c
>>> ===================================================================
>>> --- linux-3.6.orig/drivers/base/memory.c        2012-10-04 14:22:57.000000000 +0900
>>> +++ linux-3.6/drivers/base/memory.c     2012-10-04 14:45:46.653585860 +0900
>>> @@ -70,6 +70,45 @@ void unregister_memory_isolate_notifier(
>>>   }
>>>   EXPORT_SYMBOL(unregister_memory_isolate_notifier);
>>>
>>> +bool is_memblk_offline(unsigned long start, unsigned long size)
>>
>> Don't use memblk. Usually memblk mean struct numa_meminfo for x86/numa.
>> Maybe memory_range_offlined() is better.
>>
>> And, this function don't take struct memory_block, then this file may be no good
>> place.
>>
>> And you need to write down function comment.
>>
>>
>>> +{
>>> +       struct memory_block *mem = NULL;
>>> +       struct mem_section *section;
>>> +       unsigned long start_pfn, end_pfn;
>>> +       unsigned long pfn, section_nr;
>>> +
>>> +       start_pfn = PFN_DOWN(start);
>>> +       end_pfn = PFN_UP(start + size);
>>> +
>>> +       for (pfn = start_pfn; pfn<  end_pfn; pfn += PAGES_PER_SECTION) {
>>> +               section_nr = pfn_to_section_nr(pfn);
>>> +               if (!present_section_nr(section_nr))
>>> +                       continue;
>>> +
>>> +               section = __nr_to_section(section_nr);
>>> +               /* same memblock? */
>>> +               if (mem)
>>> +                       if ((section_nr>= mem->start_section_nr)&&
>>> +                           (section_nr<= mem->end_section_nr))
>>> +                               continue;
>>> +
>>> +               mem = find_memory_block_hinted(section, mem);
>>> +               if (!mem)
>>> +                       continue;
>>> +               if (mem->state == MEM_OFFLINE)
>>> +                       continue;
>>> +
>>> +               kobject_put(&mem->dev.kobj);
>>> +               return false;
>>> +       }
>>> +
>>> +       if (mem)
>>> +               kobject_put(&mem->dev.kobj);
>>> +
>>> +       return true;
>>> +}
>>> +EXPORT_SYMBOL(is_memblk_offline);
>>> +
>>>   /*
>>>    * register_memory - Setup a sysfs device for a memory block
>>>    */
>>> Index: linux-3.6/include/linux/memory.h
>>> ===================================================================
>>> --- linux-3.6.orig/include/linux/memory.h       2012-10-02 18:00:22.000000000 +0900
>>> +++ linux-3.6/include/linux/memory.h    2012-10-04 14:44:40.902581028 +0900
>>> @@ -106,6 +106,10 @@ static inline int memory_isolate_notify(
>>>   {
>>>          return 0;
>>>   }
>>> +static inline bool is_memblk_offline(unsigned long start, unsigned long size)
>>> +{
>>> +       return false;
>>> +}
>>>   #else
>>>   extern int register_memory_notifier(struct notifier_block *nb);
>>>   extern void unregister_memory_notifier(struct notifier_block *nb);
>>> @@ -120,6 +124,7 @@ extern int memory_isolate_notify(unsigne
>>>   extern struct memory_block *find_memory_block_hinted(struct mem_section *,
>>>                                                          struct memory_block *);
>>>   extern struct memory_block *find_memory_block(struct mem_section *);
>>> +extern bool is_memblk_offline(unsigned long start, unsigned long size);
>>>   #define CONFIG_MEM_BLOCK_SIZE  (PAGES_PER_SECTION<<PAGE_SHIFT)
>>>   enum mem_add_context { BOOT, HOTPLUG };
>>>   #endif /* CONFIG_MEMORY_HOTPLUG_SPARSE */
>>> Index: linux-3.6/mm/memory_hotplug.c
>>> ===================================================================
>>> --- linux-3.6.orig/mm/memory_hotplug.c  2012-10-04 14:31:08.000000000 +0900
>>> +++ linux-3.6/mm/memory_hotplug.c       2012-10-04 14:58:22.449687986 +0900
>>> @@ -1045,8 +1045,21 @@ int offline_memory(u64 start, u64 size)
>>>
>>>   int remove_memory(int nid, u64 start, u64 size)
>>>   {
>>
>> Your remove_memory() don't remove anything. that's strange.

IIUC, this batch is based on another patchset.

>>
>>
>>> -       /* It is not implemented yet*/
>>> -       return 0;
>>> +       int ret = 0;
>>> +       lock_memory_hotplug();
>>> +       /*
>>> +        * The memory might become online by other task, even if you offine it.
>>> +        * So we check whether the memory has been onlined or not.
>>> +        */
>>> +       if (!is_memblk_offline(start, size)) {
>>> +               pr_warn("memory removing [mem %#010llx-%#010llx] failed, "
>>> +                       "because the memmory range is online\n",
>>> +                       start, start + size);
>>
>> No good warning. You should output which memory block can't be
>> offlined, I think.

OK. I'll update it.

Thanks
Wen Congyang

>>
>>
>>> +               ret = -EAGAIN;
>>> +       }
>>> +
>>> +       unlock_memory_hotplug();
>>> +       return ret;
>>>   }
>>>   EXPORT_SYMBOL_GPL(remove_memory);
>>>   #else
>>>
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>>
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