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Subject[PATCH] mm/page_alloc: use ac->high_zoneidx for classzone_idx
Date
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Currently, we use the zone index of preferred_zone which represents
the best matching zone for allocation, as classzone_idx. It has a problem
on NUMA system with ZONE_MOVABLE.

In NUMA system, it can be possible that each node has different populated
zones. For example, node 0 could have DMA/DMA32/NORMAL/MOVABLE zone and
node 1 could have only NORMAL zone. In this setup, allocation request
initiated on node 0 and the one on node 1 would have different
classzone_idx, 3 and 2, respectively, since their preferred_zones are
different. If they are handled by only their own node, there is no problem.
However, if they are somtimes handled by the remote node, the problem
would happen.

In the following setup, allocation initiated on node 1 will have some
precedence than allocation initiated on node 0 when former allocation is
processed on node 0 due to not enough memory on node 1. They will have
different lowmem reserve due to their different classzone_idx thus
an watermark bars are also different.

root@ubuntu:/sys/devices/system/memory# cat /proc/zoneinfo
Node 0, zone DMA
per-node stats
...
pages free 3965
min 5
low 8
high 11
spanned 4095
present 3998
managed 3977
protection: (0, 2961, 4928, 5440)
...
Node 0, zone DMA32
pages free 757955
min 1129
low 1887
high 2645
spanned 1044480
present 782303
managed 758116
protection: (0, 0, 1967, 2479)
...
Node 0, zone Normal
pages free 459806
min 750
low 1253
high 1756
spanned 524288
present 524288
managed 503620
protection: (0, 0, 0, 4096)
...
Node 0, zone Movable
pages free 130759
min 195
low 326
high 457
spanned 1966079
present 131072
managed 131072
protection: (0, 0, 0, 0)
...
Node 1, zone DMA
pages free 0
min 0
low 0
high 0
spanned 0
present 0
managed 0
protection: (0, 0, 1006, 1006)
Node 1, zone DMA32
pages free 0
min 0
low 0
high 0
spanned 0
present 0
managed 0
protection: (0, 0, 1006, 1006)
Node 1, zone Normal
per-node stats
...
pages free 233277
min 383
low 640
high 897
spanned 262144
present 262144
managed 257744
protection: (0, 0, 0, 0)
...
Node 1, zone Movable
pages free 0
min 0
low 0
high 0
spanned 262144
present 0
managed 0
protection: (0, 0, 0, 0)

min watermark for NORMAL zone on node 0
allocation initiated on node 0: 750 + 4096 = 4846
allocation initiated on node 1: 750 + 0 = 750

This watermark difference could cause too many numa_miss allocation
in some situation and then performance could be downgraded.

Recently, there was a regression report about this problem on CMA patches
since CMA memory are placed in ZONE_MOVABLE by those patches. I checked
that problem is disappeared with this fix that uses high_zoneidx
for classzone_idx.

http://lkml.kernel.org/r/20180102063528.GG30397@yexl-desktop

Using high_zoneidx for classzone_idx is more consistent way than previous
approach because system's memory layout doesn't affect anything to it.
With this patch, both classzone_idx on above example will be 3 so will
have the same min watermark.

allocation initiated on node 0: 750 + 4096 = 4846
allocation initiated on node 1: 750 + 4096 = 4846

Reported-by: Ye Xiaolong <xiaolong.ye@intel.com>
Tested-by: Ye Xiaolong <xiaolong.ye@intel.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
---
mm/internal.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/internal.h b/mm/internal.h
index 228dd66..e1d7376 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -123,7 +123,7 @@ struct alloc_context {
bool spread_dirty_pages;
};

-#define ac_classzone_idx(ac) zonelist_zone_idx(ac->preferred_zoneref)
+#define ac_classzone_idx(ac) (ac->high_zoneidx)

/*
* Locate the struct page for both the matching buddy in our
--
2.7.4
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