Re: [PATCH] mm: fix huge page table not free after memory unplug

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 21 Dec 2025 23:11:17 -0500 Yuan Liu <yuan1.liu@intel.com> wrote:

> newly plugged memory is marked as prot_sethuge via phys_pmd_init
> without setting PG_head. During memory unplug, free_hugepage_table
> frees the page table as 2M, but pagetable_free handles it as 4K.
> 
> The following test case of memory unplug for a VM [1], tested in
> the environment [2], show that results.
> 
> +-----------------------+------+------+
> |Check System Memory    |Plug  |Unplug|
> |via free -h            |256GB |256GB |
> +-----------------------+------+------+
> | Free 4K page table    |257GB |5.6GB |
> +-----------------------+------+------+
> | Free 2M page table    |257GB |1.7GB |
> +-----------------------+------+------+
> 
> [1] Qemu commands to unhotplug 256G memory for a VM:
>     object_add memory-backend-ram,id=hotmem0,size=256G,share=on
>     device_add virtio-mem-pci,id=vmem1,memdev=hotmem0,bus=port1
>     qom-set vmem1 requested-size 256G (Plug Memory)
>     qom-set vmem1 requested-size 0G (Unplug Memory)
> 
> [2] Hardware     : Intel Icelake server
>     Guest Kernel : v6.19-rc1
>     Qemu         : v9.0.0
> 
> Launch VM:
>     qemu-system-x86_64 -accel kvm -cpu host \
>     -drive file=./Centos10_cloud.qcow2,format=qcow2,if=virtio \
>     -drive file=./seed.img,format=raw,if=virtio \
>     -smp 3,cores=3,threads=1,sockets=1,maxcpus=3 \
>     -m 2G,slots=10,maxmem=2052472M \
>     -device pcie-root-port,id=port1,bus=pcie.0,slot=1,multifunction=on \
>     -device pcie-root-port,id=port2,bus=pcie.0,slot=2 \
>     -nographic -machine q35 \
>     -nic user,hostfwd=tcp::3000-:22
> 
>    Guest kernel auto-onlines newly added memory blocks:
>    echo online > /sys/devices/system/memory/auto_online_blocks
> 
> ...
>
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1028,7 +1028,7 @@ static void __meminit free_pagetable(struct page *page, int order)
>  		free_reserved_pages(page, nr_pages);
>  #endif
>  	} else {
> -		pagetable_free(page_ptdesc(page));
> +		__free_pages(page, order);
>  	}
>  }

This reverts half of bf9e4e30f353 ("x86/mm: use pagetable_free()").

What about the other half?  The below change that patch made to
arch/x86/mm/pat/set_memory.c - is that OK?

--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -429,7 +429,7 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
 
 	list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
 		list_del(&ptdesc->pt_list);
-		__free_page(ptdesc_page(ptdesc));
+		pagetable_free(ptdesc);
 	}
 }