Subject: Re: [PATCH v6 00/15] memory-hotplug: hot-remove physical memory

From: Tang Chen <[email-masked]>
Date: Wed, 30 Jan 2013 18:15:15 +0800

Hi Simon,

Please see below. :)

On 01/29/2013 08:52 PM, Simon Jeons wrote:
> Hi Tang,
>
> On Wed, 2013-01-09 at 17:32 +0800, Tang Chen wrote:
>> Here is the physical memory hot-remove patch-set based on 3.8rc-2.
>
> Some questions ask you, not has relationship with this patchset, but is
> memory hotplug stuff.
>
> 1. In function node_states_check_changes_online:
>
> comments:
> * If we don't have HIGHMEM nor movable node,
> * node_states[N_NORMAL_MEMORY] contains nodes which have zones of
> * 0...ZONE_MOVABLE, set zone_last to ZONE_MOVABLE.
>
> How to understand it? Why we don't have HIGHMEM nor movable node and
> node_staes[N_NORMAL_MEMORY] contains 0...ZONE_MOVABLE, IIUC,
> N_NORMAL_MEMORY only means the node has regular memory.
>

First of all, I think we need to understand why we need N_MEMORY.

In order to support movable node, which has only ZONE_MOVABLE (the last 
zone),
we introduce N_MEMORY to represent the node has normal, highmem and 
movable memory.

Here, "we have movable node" means you configured CONFIG_MOVABLE_NODE.
This config option doesn't mean we don't have movable pages, (NO)
it means we don't have a node which has only movable pages (only have 
ZONE_MOVABLE). (YES)

Here, if we don't have CONFIG_MOVABLE_NODE (we don't have movable node), 
we don't need a
separate node_states[] element to represent a particular node because we 
won't have a node
which has only ZONE_MOVABLE.

So,
1) if we don't have highmem nor movable node, N_MEMORY == N_HIGH_MEMORY 
== N_NORMAL_MEMORY,
    which means N_NORMAL_MEMORY effects as N_MEMORY. If we online pages 
as movable, we need
    to update node_states[N_NORMAL_MEMORY].

Please refer to the definition of enum zone_type, if we don't have 
CONFIG_HIGHMEM, we won't
have ZONE_HIGHMEM, but ZONE_NORMAL and ZONE_MOVABLE will always there. 
So we can have movable
pages, and the zone_last should be ZONE_MOVABLE.

Again, because we won't have a node only having ZONE_MOVABLE, so we just 
need to update
node_states[N_NORMAL_MEMORY].

> * If we don't have movable node, node_states[N_NORMAL_MEMORY]
> * contains nodes which have zones of 0...ZONE_MOVABLE,
> * set zone_last to ZONE_MOVABLE.
>
> How to understand?

2) this code is in #ifdef CONFIG_HIGHMEM, which means we have highmem, 
so if we don't have
    movable node, N_MEMORY == N_HIGH_MEMORY, and N_HIGH_MEMORY effects 
as N_MEMORY. If we
    online pages as movable, we need to update node_states[N_NORMAL_MEMORY].

>
> 2. In function move_pfn_range_left, why end<= z2->zone_start_pfn is not
> correct? The comments said that must include/overlap, why?
>

This one is easy, if I understand you correctly.
move_pfn_range_left() is used to move the left most part [start_pfn, 
end_pfn) of z2 to z1.
So if end_pfn<= z2->zone_start_pfn, it means [start_pfn, end_pfn) is not 
part of z2.
Then it fails.

> 3. In function online_pages, the normal case(w/o online_kenrel,
> online_movable), why not check if the new zone is overlap with adjacent
> zones?
>

Can a zone overlap with the others ? I don't think so.

One pfn could only be in one zone,
    zone = page_zone(pfn_to_page(pfn));

it could overlap with others, I think. :)

But maybe I misunderstand you. :)

> 4. Could you summarize the difference implementation between hot-add and
> logic-add, hot-remove and logic-remove?

Sorry, I don't quite understand what do you mean by logic-add/remove.
Would you please explain more ?

If you meant the sys fs interfaces, I think they are just another set of 
entrances
of memory hotplug.

Thanks.  :)

>
>
>>
>> This patch-set aims to implement physical memory hot-removing.
>>
>> The patches can free/remove the following things:
>>
>>    - /sys/firmware/memmap/X/{end, start, type} : [PATCH 4/15]
>>    - memmap of sparse-vmemmap                  : [PATCH 6,7,8,10/15]
>>    - page table of removed memory              : [RFC PATCH 7,8,10/15]
>>    - node and related sysfs files              : [RFC PATCH 13-15/15]
>>
>>
>> Existing problem:
>> If CONFIG_MEMCG is selected, we will allocate memory to store page cgroup
>> when we online pages.
>>
>> For example: there is a memory device on node 1. The address range
>> is [1G, 1.5G). You will find 4 new directories memory8, memory9, memory10,
>> and memory11 under the directory /sys/devices/system/memory/.
>>
>> If CONFIG_MEMCG is selected, when we online memory8, the memory stored page
>> cgroup is not provided by this memory device. But when we online memory9, the
>> memory stored page cgroup may be provided by memory8. So we can't offline
>> memory8 now. We should offline the memory in the reversed order.
>>
>> When the memory device is hotremoved, we will auto offline memory provided
>> by this memory device. But we don't know which memory is onlined first, so
>> offlining memory may fail.
>>
>> In patch1, we provide a solution which is not good enough:
>> Iterate twice to offline the memory.
>> 1st iterate: offline every non primary memory block.
>> 2nd iterate: offline primary (i.e. first added) memory block.
>>
>> And a new idea from Wen Congyang<[email-masked]>  is:
>> allocate the memory from the memory block they are describing.
>>
>> But we are not sure if it is OK to do so because there is not existing API
>> to do so, and we need to move page_cgroup memory allocation from MEM_GOING_ONLINE
>> to MEM_ONLINE. And also, it may interfere the hugepage.
>>
>>
>>
>> How to test this patchset?
>> 1. apply this patchset and build the kernel. MEMORY_HOTPLUG, MEMORY_HOTREMOVE,
>>     ACPI_HOTPLUG_MEMORY must be selected.
>> 2. load the module acpi_memhotplug
>> 3. hotplug the memory device(it depends on your hardware)
>>     You will see the memory device under the directory /sys/bus/acpi/devices/.
>>     Its name is PNP0C80:XX.
>> 4. online/offline pages provided by this memory device
>>     You can write online/offline to /sys/devices/system/memory/memoryX/state to
>>     online/offline pages provided by this memory device
>> 5. hotremove the memory device
>>     You can hotremove the memory device by the hardware, or writing 1 to
>>     /sys/bus/acpi/devices/PNP0C80:XX/eject.
>
> Is there a similar knode to hot-add the memory device?
>
>>
>>
>> Note: if the memory provided by the memory device is used by the kernel, it
>> can't be offlined. It is not a bug.
>>
>>
>> Changelogs from v5 to v6:
>>   Patch3: Add some more comments to explain memory hot-remove.
>>   Patch4: Remove bootmem member in struct firmware_map_entry.
>>   Patch6: Repeatedly register bootmem pages when using hugepage.
>>   Patch8: Repeatedly free bootmem pages when using hugepage.
>>   Patch14: Don't free pgdat when offlining a node, just reset it to 0.
>>   Patch15: New patch, pgdat is not freed in patch14, so don't allocate a new
>>            one when online a node.
>>
>> Changelogs from v4 to v5:
>>   Patch7: new patch, move pgdat_resize_lock into sparse_remove_one_section() to
>>           avoid disabling irq because we need flush tlb when free pagetables.
>>   Patch8: new patch, pick up some common APIs that are used to free direct mapping
>>           and vmemmap pagetables.
>>   Patch9: free direct mapping pagetables on x86_64 arch.
>>   Patch10: free vmemmap pagetables.
>>   Patch11: since freeing memmap with vmemmap has been implemented, the config
>>            macro CONFIG_SPARSEMEM_VMEMMAP when defining __remove_section() is
>>            no longer needed.
>>   Patch13: no need to modify acpi_memory_disable_device() since it was removed,
>>            and add nid parameter when calling remove_memory().
>>
>> Changelogs from v3 to v4:
>>   Patch7: remove unused codes.
>>   Patch8: fix nr_pages that is passed to free_map_bootmem()
>>
>> Changelogs from v2 to v3:
>>   Patch9: call sync_global_pgds() if pgd is changed
>>   Patch10: fix a problem int the patch
>>
>> Changelogs from v1 to v2:
>>   Patch1: new patch, offline memory twice. 1st iterate: offline every non primary
>>           memory block. 2nd iterate: offline primary (i.e. first added) memory
>>           block.
>>
>>   Patch3: new patch, no logical change, just remove reduntant codes.
>>
>>   Patch9: merge the patch from wujianguo into this patch. flush tlb on all cpu
>>           after the pagetable is changed.
>>
>>   Patch12: new patch, free node_data when a node is offlined.
>>
>>
>> Tang Chen (6):
>>    memory-hotplug: move pgdat_resize_lock into
>>      sparse_remove_one_section()
>>    memory-hotplug: remove page table of x86_64 architecture
>>    memory-hotplug: remove memmap of sparse-vmemmap
>>    memory-hotplug: Integrated __remove_section() of
>>      CONFIG_SPARSEMEM_VMEMMAP.
>>    memory-hotplug: remove sysfs file of node
>>    memory-hotplug: Do not allocate pdgat if it was not freed when
>>      offline.
>>
>> Wen Congyang (5):
>>    memory-hotplug: try to offline the memory twice to avoid dependence
>>    memory-hotplug: remove redundant codes
>>    memory-hotplug: introduce new function arch_remove_memory() for
>>      removing page table depends on architecture
>>    memory-hotplug: Common APIs to support page tables hot-remove
>>    memory-hotplug: free node_data when a node is offlined
>>
>> Yasuaki Ishimatsu (4):
>>    memory-hotplug: check whether all memory blocks are offlined or not
>>      when removing memory
>>    memory-hotplug: remove /sys/firmware/memmap/X sysfs
>>    memory-hotplug: implement register_page_bootmem_info_section of
>>      sparse-vmemmap
>>    memory-hotplug: memory_hotplug: clear zone when removing the memory
>>
>>   arch/arm64/mm/mmu.c                  |    3 +
>>   arch/ia64/mm/discontig.c             |   10 +
>>   arch/ia64/mm/init.c                  |   18 ++
>>   arch/powerpc/mm/init_64.c            |   10 +
>>   arch/powerpc/mm/mem.c                |   12 +
>>   arch/s390/mm/init.c                  |   12 +
>>   arch/s390/mm/vmem.c                  |   10 +
>>   arch/sh/mm/init.c                    |   17 ++
>>   arch/sparc/mm/init_64.c              |   10 +
>>   arch/tile/mm/init.c                  |    8 +
>>   arch/x86/include/asm/pgtable_types.h |    1 +
>>   arch/x86/mm/init_32.c                |   12 +
>>   arch/x86/mm/init_64.c                |  390 +++++++++++++++++++++++++++++
>>   arch/x86/mm/pageattr.c               |   47 ++--
>>   drivers/acpi/acpi_memhotplug.c       |    8 +-
>>   drivers/base/memory.c                |    6 +
>>   drivers/firmware/memmap.c            |   96 +++++++-
>>   include/linux/bootmem.h              |    1 +
>>   include/linux/firmware-map.h         |    6 +
>>   include/linux/memory_hotplug.h       |   15 +-
>>   include/linux/mm.h                   |    4 +-
>>   mm/memory_hotplug.c                  |  459 +++++++++++++++++++++++++++++++---
>>   mm/sparse.c                          |    8 +-
>>   23 files changed, 1094 insertions(+), 69 deletions(-)
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to [email-masked].  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email:<a href=mailto:"[email-masked]">  [email-masked]</a>
>
>
>