[PATCH -v4 0/14] mm: make movable onlining suck less

From: Michal Hocko
Date: Mon May 15 2017 - 04:59:14 EST

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Hi,
The last version of this series has been posted here [1]. The timing wasn't
all that great so this is mostly a resubmit. I've added one additional patch
to fix another pfn walker noticed by Joonsoo (this is patch 11) and also
added a clarification for pfn_valid() and offline pages.

There is still a lot of work on top - namely this implementation doesn't
support reonlining to a different zone on the zones boundaries but I
will do that in a separate series because this one is getting quite
large already and it should work reasonably well now.

Joonsoo had some worries about pfn_valid and suggested to change its
semantic to return false on offline holes but I would be rally worried
to change a established semantic used by a lot of code and so I have
introuduced pfn_to_online_page helper instead. If this is seen as a
controversial point I would rather drop pfn_to_online_page and related
patches as they are not stictly necessary because the code would be
similarly broken as now wrt. offline holes.

This is a rebase on top of the current mmotm tree (mmotm-2017-05-12-15-53)
and the full series is in
git://git.kernel.org/pub/scm/linux/kernel/git/mhocko/mm.git try
attempts/rewrite-mem_hotplug branch.

Motivation:
Movable onlining is a real hack with many downsides - mainly
reintroduction of lowmem/highmem issues we used to have on 32b systems -
but it is the only way to make the memory hotremove more reliable which
is something that people are asking for.

The current semantic of memory movable onlinening is really cumbersome,
however. The main reason for this is that the udev driven approach is
basically unusable because udev races with the memory probing while only
the last memory block or the one adjacent to the existing zone_movable
are allowed to be onlined movable. In short the criterion for the
successful online_movable changes under udev's feet. A reliable udev
approach would require a 2 phase approach where the first successful
movable online would have to check all the previous blocks and online
them in descending order. This is hard to be considered sane.

This patchset aims at making the onlining semantic more usable. First of
all it allows to online memory movable as long as it doesn't clash with
the existing ZONE_NORMAL. That means that ZONE_NORMAL and ZONE_MOVABLE
cannot overlap. Currently I preserve the original ordering semantic so
the zone always precedes the movable zone but I have plans to remove this
restriction in future because it is not really necessary.

First 3 patches are cleanups which should be ready to be merged right
away (unless I have missed something subtle of course).

Patch 4 deals with ZONE_DEVICE dependencies down the __add_pages path.

Patch 5 deals with implicit assumptions of register_one_node on pgdat
initialization.

Patches 6-10 deal with offline holes in the zone for pfn walkers. I
hope I got all of them right but people familiar with compaction should
double check this.

Patch 11 is the core of the change. In order to make it easier to review
I have tried it to be as minimalistic as possible and the large code
removal is moved to patch 14.

Patch 12 is a trivial follow up cleanup. Patch 13 fixes sparse warnings
and finally patch 14 removes the unused code.

I have tested the patches in kvm:
# qemu-system-x86_64 -enable-kvm -monitor pty -m 2G,slots=4,maxmem=4G -numa node,mem=1G -numa node,mem=1G ...

and then probed the additional memory by
(qemu) object_add memory-backend-ram,id=mem1,size=1G
(qemu) device_add pc-dimm,id=dimm1,memdev=mem1

Then I have used this simple script to probe the memory block by hand
# cat probe_memblock.sh
#!/bin/sh

BLOCK_NR=$1

# echo $((0x100000000+$BLOCK_NR*(128<<20))) > /sys/devices/system/memory/probe

# for i in $(seq 10); do sh probe_memblock.sh $i; done
# grep . /sys/devices/system/memory/memory3?/valid_zones 2>/dev/null
/sys/devices/system/memory/memory33/valid_zones:Normal Movable
/sys/devices/system/memory/memory34/valid_zones:Normal Movable
/sys/devices/system/memory/memory35/valid_zones:Normal Movable
/sys/devices/system/memory/memory36/valid_zones:Normal Movable
/sys/devices/system/memory/memory37/valid_zones:Normal Movable
/sys/devices/system/memory/memory38/valid_zones:Normal Movable
/sys/devices/system/memory/memory39/valid_zones:Normal Movable

The main difference to the original implementation is that all new
memblocks can be both online_kernel and online_movable initially
because there is no clash obviously. For the comparison the original
implementation would have

/sys/devices/system/memory/memory33/valid_zones:Normal
/sys/devices/system/memory/memory34/valid_zones:Normal
/sys/devices/system/memory/memory35/valid_zones:Normal
/sys/devices/system/memory/memory36/valid_zones:Normal
/sys/devices/system/memory/memory37/valid_zones:Normal
/sys/devices/system/memory/memory38/valid_zones:Normal
/sys/devices/system/memory/memory39/valid_zones:Normal Movable

Now
# echo online_movable > /sys/devices/system/memory/memory34/state
# grep . /sys/devices/system/memory/memory3?/valid_zones 2>/dev/null
/sys/devices/system/memory/memory33/valid_zones:Normal Movable
/sys/devices/system/memory/memory34/valid_zones:Movable
/sys/devices/system/memory/memory35/valid_zones:Movable
/sys/devices/system/memory/memory36/valid_zones:Movable
/sys/devices/system/memory/memory37/valid_zones:Movable
/sys/devices/system/memory/memory38/valid_zones:Movable
/sys/devices/system/memory/memory39/valid_zones:Movable

Block 33 can still be online both kernel and movable while all
the remaining can be only movable.
/proc/zonelist says
Node 0, zone Normal
pages free 0
min 0
low 0
high 0
spanned 0
present 0
--
Node 0, zone Movable
pages free 32753
min 85
low 117
high 149
spanned 32768
present 32768

A new memblock at a lower address will result in a new memblock (32)
which will still allow both Normal and Movable.
# sh probe_memblock.sh 0
# grep . /sys/devices/system/memory/memory3[2-5]/valid_zones 2>/dev/null
/sys/devices/system/memory/memory32/valid_zones:Normal Movable
/sys/devices/system/memory/memory33/valid_zones:Normal Movable
/sys/devices/system/memory/memory34/valid_zones:Movable
/sys/devices/system/memory/memory35/valid_zones:Movable

and online_kernel will convert it to the zone normal properly
while 33 can be still onlined both ways.
# echo online_kernel > /sys/devices/system/memory/memory32/state
# grep . /sys/devices/system/memory/memory3[2-5]/valid_zones 2>/dev/null
/sys/devices/system/memory/memory32/valid_zones:Normal
/sys/devices/system/memory/memory33/valid_zones:Normal Movable
/sys/devices/system/memory/memory34/valid_zones:Movable
/sys/devices/system/memory/memory35/valid_zones:Movable

/proc/zoneinfo will now tell
Node 0, zone Normal
pages free 65441
min 165
low 230
high 295
spanned 65536
present 65536
--
Node 0, zone Movable
pages free 32740
min 82
low 114
high 146
spanned 32768
present 32768

so both zones have one memblock spanned and present.

Onlining 39 should associate this block to the movable zone
# echo online > /sys/devices/system/memory/memory39/state

/proc/zoneinfo will now tell
Node 0, zone Normal
pages free 32765
min 80
low 112
high 144
spanned 32768
present 32768
--
Node 0, zone Movable
pages free 65501
min 160
low 225
high 290
spanned 196608
present 65536

so we will have a movable zone which spans 6 memblocks, 2 present and 4
representing a hole.

Offlining both movable blocks will lead to the zone with no present
pages which is the expected behavior I believe.
# echo offline > /sys/devices/system/memory/memory39/state
# echo offline > /sys/devices/system/memory/memory34/state
# grep -A6 "Movable\|Normal" /proc/zoneinfo
Node 0, zone Normal
pages free 32735
min 90
low 122
high 154
spanned 32768
present 32768
--
Node 0, zone Movable
pages free 0
min 0
low 0
high 0
spanned 196608
present 0

Any thoughts, complains, suggestions?

As a bonus we will get a nice cleanup in the memory hotplug codebase.
arch/ia64/mm/init.c | 11 +-
arch/powerpc/mm/mem.c | 12 +-
arch/s390/mm/init.c | 32 +--
arch/sh/mm/init.c | 10 +-
arch/x86/mm/init_32.c | 7 +-
arch/x86/mm/init_64.c | 11 +-
drivers/base/memory.c | 79 +++----
drivers/base/node.c | 58 ++----
include/linux/memory_hotplug.h | 40 +++-
include/linux/mmzone.h | 57 +++++-
include/linux/node.h | 35 +++-
kernel/memremap.c | 6 +-
mm/compaction.c | 5 +-
mm/memory_hotplug.c | 455 ++++++++++++++---------------------------
mm/page_alloc.c | 13 +-
mm/page_isolation.c | 26 ++-
mm/sparse.c | 48 ++++-
mm/vmstat.c | 4 +-
18 files changed, 417 insertions(+), 492 deletions(-)

Shortlog says:
Michal Hocko (14):
mm: remove return value from init_currently_empty_zone
mm, memory_hotplug: use node instead of zone in can_online_high_movable
mm: drop page_initialized check from get_nid_for_pfn
mm, memory_hotplug: get rid of is_zone_device_section
mm, memory_hotplug: split up register_one_node
mm, memory_hotplug: consider offline memblocks removable
mm: consider zone which is not fully populated to have holes
mm, compaction: skip over holes in __reset_isolation_suitable
mm: __first_valid_page skip over offline pages
mm, vmstat: skip reporting offline pages in pagetypeinfo
mm, memory_hotplug: do not associate hotadded memory to zones until online
mm, memory_hotplug: replace for_device by want_memblock in arch_add_memory
mm, memory_hotplug: fix the section mismatch warning
mm, memory_hotplug: remove unused cruft after memory hotplug rework

[1] http://lkml.kernel.org/r/20170421120512.23960-1-mhocko@xxxxxxxxxx

Next message: Michal Hocko: "[PATCH 01/14] mm: remove return value from init_currently_empty_zone"
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