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.
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.
-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 \
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
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