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Subject[PATCH 0/6] mm: make movable onlining suck less
Date
Hi,
I have posted a crude RFC for this rework [1] and there didn't seem any
objections so I have split up the patch into smaller chunks which will
make the review easier hopefully.

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.

The series consists of 4 cleanup patches which should be ready to be
merged right away (unless I have missed something subtle of course).

Patch 5 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 6.

I would appreciate if s390 folks could take a look at patch 4 and the
arch_add_memory because I am not sure I've grokked what they wanted to
achieve there completely.

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 | 10 +-
arch/powerpc/mm/mem.c | 12 +-
arch/s390/mm/init.c | 32 +---
arch/sh/mm/init.c | 10 +-
arch/tile/mm/init.c | 30 ---
arch/x86/mm/init_32.c | 7 +-
arch/x86/mm/init_64.c | 11 +-
drivers/base/memory.c | 52 ++---
include/linux/memory_hotplug.h | 17 +-
include/linux/mmzone.h | 13 +-
kernel/memremap.c | 5 +-
mm/memory_hotplug.c | 423 ++++++++++++-----------------------------
mm/page_alloc.c | 11 +-
mm/sparse.c | 3 +-
14 files changed, 186 insertions(+), 450 deletions(-)

Shortlog says:
Michal Hocko (6):
mm: get rid of zone_is_initialized
mm, tile: drop arch_{add,remove}_memory
mm: remove return value from init_currently_empty_zone
mm, memory_hotplug: use node instead of zone in can_online_high_movable
mm, memory_hotplug: do not associate hotadded memory to zones until online
mm, memory_hotplug: remove unused cruft after memory hotplug rework

[1] http://lkml.kernel.org/r/20170315091347.GA32626@dhcp22.suse.cz

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