linux-kernel - Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as

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Message-ID: <aOgpz6no2Jx2-Y8Z@gourry-fedora-PF4VCD3F>
Date: Thu, 9 Oct 2025 17:31:59 -0400
From: Gregory Price <gourry@...rry.net>
To: David Hildenbrand <david@...hat.com>
Cc: Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org, corbet@....net,
	muchun.song@...ux.dev, osalvador@...e.de, akpm@...ux-foundation.org,
	hannes@...xchg.org, laoar.shao@...il.com, brauner@...nel.org,
	mclapinski@...gle.com, joel.granados@...nel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Mel Gorman <mgorman@...e.de>,
	Alexandru Moise <00moses.alexander00@...il.com>,
	Mike Kravetz <mike.kravetz@...cle.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as_movable
 sysctl"

On Thu, Oct 09, 2025 at 08:51:54PM +0200, David Hildenbrand wrote:
> On 09.10.25 17:29, Gregory Price wrote:
> Or would it be sufficient to selectively enable (explicit opt-in) some user
> pages to end up on ZONE_MOVABLE? IOW, change the semantics of the zone by an
> admin.
> 
> Like, allowing longterm pinning on ZONE_MOVABLE.
> 
> Sure, it would degrade memory hotunplug (until the relevant applications are
> shut down) and probably some other things.
> 
> Further, I am not so sure about the value of having ZONE_MOVABLE sprinkled
> with small unmovable allocations (same concern regarding any such zone that
> allows for unmovable things). Kind of against the whole concept.
> 
> But I mean, if the admin decides to do that (opt in), so he is to blame.
> 

For what it's worth, this patch (or the new one i posted as an RFC), I
was able to allocate gigantic pages and migrate them back and forth
between nodes even after they were allocated for KVM instances.

I was surprised this did not cause pinning.

This was all while running the QEMU machine actively eating ~2GB of
memory.  So this seems... acceptable?  My primary use case was VM
hugepages, but it doesn't even seem like these have been pinned.

I think the confidential-compute / guest_memfd path would have an
issue, because those are pinned and/or entirely unmapped from the
host, but that just seems like a known quantity and a reason to leave
this off by default (make them read the docs :]).

Seems like this is pretty stable tbh.  Obviously if you hack off the
node0 hugepages migration fails - but I feel like you're signing up for
that when you turn the bit on.

Test I ran is below.

~Gregory

---
Host allocates hugepages, runs a qemu image with numa structure, and
migrates the huge pages back and forth

cat /proc/sys/vm/movable_gigantic_pages 
1
cat .../node0/hugepages/hugepages-1048576kB/nr_hugepages
24
cat .../node1/hugepages/hugepages-1048576kB/nr_hugepages
12

qemu-system-x86_64 \
  -machine q35,accel=kvm \
  -cpu host \
  -smp 8,sockets=1,cores=8,threads=1 \
  -m 2G \
  -mem-prealloc \
  -object memory-backend-file,id=mem0,mem-path=/dev/hugepages,prealloc=on,size=1G,host-nodes=0,policy=bind \
  -object memory-backend-file,id=mem1,mem-path=/dev/hugepages,prealloc=on,size=1G,host-nodes=1,policy=bind \
  -numa node,nodeid=0,cpus=0-7,memdev=mem0 \
  -numa node,nodeid=1,memdev=mem1 \
  -nographic \
  -drive file=fedora/hdd.qcow2 \
  -cdrom fedora/seedci.iso

grep bind /proc/1041805/numa_maps
7efc80000000 bind:1 file=/dev/hugepages/qemu_back_mem.mem1.xKNv1N\040(deleted) huge anon=1 dirty=1 N1=1 kernelpagesize_kB=1048576
7efd00000000 bind:0 file=/dev/hugepages/qemu_back_mem.mem0.E19dYs\040(deleted) huge anon=1 dirty=1 N0=1 kernelpagesize_kB=1048576

# Move both to node 0  (uses move_pages(pid, ...)
./move.sh 0
Pages migrated successfully
status[0]: 0
Pages migrated successfully
status[0]: 0

grep bind /proc/1041805/numa_maps
7efc80000000 bind:1 file=/dev/hugepages/qemu_back_mem.mem1.xKNv1N\040(deleted) huge anon=1 dirty=1 N0=1 kernelpagesize_kB=1048576
7efd00000000 bind:0 file=/dev/hugepages/qemu_back_mem.mem0.E19dYs\040(deleted) huge anon=1 dirty=1 N0=1 kernelpagesize_kB=1048576

# Move both to node 1
./move.sh 1
Pages migrated successfully
status[0]: 1
Pages migrated successfully
status[0]: 1

grep bind /proc/1041805/numa_maps 
7efc80000000 bind:1 file=/dev/hugepages/qemu_back_mem.mem1.xKNv1N\040(deleted) huge anon=1 dirty=1 N1=1 kernelpagesize_kB=1048576
7efd00000000 bind:0 file=/dev/hugepages/qemu_back_mem.mem0.E19dYs\040(deleted) huge anon=1 dirty=1 N1=1 kernelpagesize_kB=1048576

---
Guest
Running python script that eats 1.7GB of memory

import time

# 1.2 GB in bytes
size_in_bytes = int(1.7 * 1024 * 1024 * 1024)

# Allocate memory
data = bytearray(size_in_bytes)

print(f"Allocated {len(data) / (1024 * 1024 * 1024):.2f} GB of memory.")

# Keep the process alive so you can inspect it (e.g., with top or htop)
try:
    while True:
        print("nom")
        time.sleep(10)
except KeyboardInterrupt:
    print("Exiting.")