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Subject[PATCH 4.14 198/237] mm/vmscan.c: dont round up scan size for online memory cgroup
From: Gavin Shan <>

commit 76073c646f5f4999d763f471df9e38a5a912d70d upstream.

Commit 68600f623d69 ("mm: don't miss the last page because of round-off
error") makes the scan size round up to @denominator regardless of the
memory cgroup's state, online or offline. This affects the overall
reclaiming behavior: the corresponding LRU list is eligible for
reclaiming only when its size logically right shifted by @sc->priority
is bigger than zero in the former formula.

For example, the inactive anonymous LRU list should have at least 0x4000
pages to be eligible for reclaiming when we have 60/12 for
swappiness/priority and without taking scan/rotation ratio into account.

After the roundup is applied, the inactive anonymous LRU list becomes
eligible for reclaiming when its size is bigger than or equal to 0x1000
in the same condition.

(0x4000 >> 12) * 60 / (60 + 140 + 1) = 1
((0x1000 >> 12) * 60) + 200) / (60 + 140 + 1) = 1

aarch64 has 512MB huge page size when the base page size is 64KB. The
memory cgroup that has a huge page is always eligible for reclaiming in
that case.

The reclaiming is likely to stop after the huge page is reclaimed,
meaing the further iteration on @sc->priority and the silbing and child
memory cgroups will be skipped. The overall behaviour has been changed.
This fixes the issue by applying the roundup to offlined memory cgroups
only, to give more preference to reclaim memory from offlined memory
cgroup. It sounds reasonable as those memory is unlikedly to be used by

The issue was found by starting up 8 VMs on a Ampere Mustang machine,
which has 8 CPUs and 16 GB memory. Each VM is given with 2 vCPUs and
2GB memory. It took 264 seconds for all VMs to be completely up and
784MB swap is consumed after that. With this patch applied, it took 236
seconds and 60MB swap to do same thing. So there is 10% performance
improvement for my case. Note that KSM is disable while THP is enabled
in the testing.

total used free shared buff/cache available
Mem: 16196 10065 2049 16 4081 3749
Swap: 8175 784 7391
total used free shared buff/cache available
Mem: 16196 11324 3656 24 1215 2936
Swap: 8175 60 8115

Fixes: 68600f623d69 ("mm: don't miss the last page because of round-off error")
Signed-off-by: Gavin Shan <>
Acked-by: Roman Gushchin <>
Cc: <> [4.20+]
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

mm/vmscan.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2369,10 +2369,13 @@ out:
* Scan types proportional to swappiness and
* their relative recent reclaim efficiency.
- * Make sure we don't miss the last page
- * because of a round-off error.
+ * Make sure we don't miss the last page on
+ * the offlined memory cgroups because of a
+ * round-off error.
- scan = DIV64_U64_ROUND_UP(scan * fraction[file],
+ scan = mem_cgroup_online(memcg) ?
+ div64_u64(scan * fraction[file], denominator) :
+ DIV64_U64_ROUND_UP(scan * fraction[file],

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