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

[Yasuaki Ishimatsu]

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@xxxxxxxxx> 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.