Re: [PATCH v2] memory_hotplug: zone_can_shift() returns boolean value

[Yasuaki Ishimatsu <yasu.isimatu@gmail.com>]

Hi Andrew

On 01/09/2017 06:27 PM, Andrew Morton wrote:
> On Tue, 13 Dec 2016 15:29:49 -0500 Yasuaki Ishimatsu <yasu.isimatu@gmail.com> wrote:
>
>> online_{kernel|movable} is used to change the memory zone to
>> ZONE_{NORMAL|MOVABLE} and online the memory.
>>
>> To check that memory zone can be changed, zone_can_shift() is used.
>> Currently the function returns minus integer value, plus integer
>> value and 0. When the function returns minus or plus integer value,
>> it means that the memory zone can be changed to ZONE_{NORNAL|MOVABLE}.
>>
>> But when the function returns 0, there is 2 meanings.
>>
>> One of the meanings is that the memory zone does not need to be changed.
>> For example, when memory is in ZONE_NORMAL and onlined by online_kernel
>> the memory zone does not need to be changed.
>>
>> Another meaning is that the memory zone cannot be changed. When memory
>> is in ZONE_NORMAL and onlined by online_movable, the memory zone may
>> not be changed to ZONE_MOVALBE due to memory online limitation(see
>> Documentation/memory-hotplug.txt). In this case, memory must not be
>> onlined.
>>
>> The patch changes the return type of zone_can_shift() so that memory
>> is not onlined when memory zone cannot be changed.
>
> What are the user-visible runtime effects of this fix?

The user-visible runtime effects of the fix are here:

Before applying patch:
   # grep -A 35 "Node 2" /proc/zoneinfo
   Node 2, zone   Normal
   <snip>
      node_scanned  0
           spanned  8388608
           present  7864320
           managed  7864320
   # echo online_movable > memory4097/state
   # grep -A 35 "Node 2" /proc/zoneinfo
   Node 2, zone   Normal
   <snip>
      node_scanned  0
           spanned  8388608
           present  8388608
           managed  8388608

   online_movable operation succeeded. But memory is onlined as
   ZONE_NORMAL, not ZONE_MOVABLE.

After applying patch:
   # grep -A 35 "Node 2" /proc/zoneinfo
   Node 2, zone   Normal
   <snip>
      node_scanned  0
           spanned  8388608
           present  7864320
           managed  7864320
   # echo online_movable > memory4097/state
   bash: echo: write error: Invalid argument
   # grep -A 35 "Node 2" /proc/zoneinfo
   Node 2, zone   Normal
   <snip>
      node_scanned  0
           spanned  8388608
           present  7864320
           managed  7864320

   online_movable operation failed because of failure of changing
   the memory zone from ZONE_NORMAL to ZONE_MOVABLE

> Please always include this info when fixing bugs - it is required so
> that others can decide which kernel version(s) need the fix.

I'll add the above information and resend the patch as v3.

Thanks,
Yasuaki Ishimatsu

> Thanks.
>