Re: [RFC PATCH] mm/page_reporting: Adjust threshold according to MAX_ORDER

From: David Hildenbrand
Date: Tue Jun 01 2021 - 04:01:32 EST

On 01.06.21 05:33, Gavin Shan wrote:
The PAGE_REPORTING_MIN_ORDER is equal to @pageblock_order, taken as
minimal order (threshold) to trigger page reporting. The page reporting
is never triggered with the following configurations and settings on
aarch64. In the particular scenario, the page reporting won't be triggered
until the largest (2 ^ (MAX_ORDER-1)) free area is achieved from the
page freeing. The condition is very hard, or even impossible to be met.

pageblock_order: 13

The issue can be reproduced in VM, running kernel with above configurations
and settings. The 'memhog' is used inside the VM to access 512MB anonymous
area. The QEMU's RSS doesn't drop accordingly after 'memhog' exits.

/home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
-accel kvm -machine virt,gic-version=host \
-cpu host -smp 8,sockets=2,cores=4,threads=1 -m 4096M,maxmem=64G \
-object memory-backend-ram,id=mem0,size=2048M \
-object memory-backend-ram,id=mem1,size=2048M \
-numa node,nodeid=0,cpus=0-3,memdev=mem0 \
-numa node,nodeid=1,cpus=4-7,memdev=mem1 \
: \
-device virtio-balloon-pci,id=balloon0,free-page-reporting=yes

This tries to fix the issue by adjusting the threshold to the smaller value
of @pageblock_order and (MAX_ORDER/2). With this applied, the QEMU's RSS
drops after 'memhog' exits.

IIRC, we use pageblock_order to

a) Reduce the free page reporting overhead. Reporting on small chunks can make us report constantly with little system activity.

b) Avoid splitting THP in the hypervisor, avoiding downgraded VM performance.

c) Avoid affecting creation of pageblock_order pages while hinting is active. I think there are cases where "temporary pulling sub-pageblock pages" can negatively affect creation of pageblock_order pages. Concurrent compaction would be one of these cases.

The monstrosity called aarch64 64k is really special in that sense, because a) does not apply because pageblocks are just very big, b) does sometimes not apply because either our VM isn't backed by (rare) 512MB THP or uses 4k with 2MB THP and c) similarly doesn't apply in smallish VMs because we don't really happen to create 512MB THP either way.

For example, going on x86-64 from reporting 2MB to something like 32KB is absolutely undesired.

I think if we want to go down that path (and I am not 100% sure yet if we want to), we really want to treat only the special case in a special way. Note that even when doing it only for aarch64 with 64k, you will still end up splitting THP in a hypervisor if it uses 64k base pages (b)) and can affect creation of THP, for example, when compacting (c), so there is a negative side to that.


David / dhildenb