linux-kernel - Re: [PATCH v2 0/4] Introduce movable pages for Hyper-V guests

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Message-ID: <aNyoK8xfgy2zpKCf@liuwe-devbox-ubuntu-v2.lamzopl0uupeniq2etz1fddiyg.xx.internal.cloudapp.net>
Date: Wed, 1 Oct 2025 04:03:55 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/4] Introduce movable pages for Hyper-V guests

On Mon, Sep 29, 2025 at 04:02:59PM +0000, Stanislav Kinsburskii wrote:
> From the start, the root-partition driver allocates, pins, and maps all
> guest memory into the hypervisor at guest creation. This is simple: Linux
> cannot move the pages, so the guest’s view in Linux and in Microsoft
> Hypervisor never diverges.
> 
> However, this approach has major drawbacks:
> - NUMA: affinity can’t be changed at runtime, so you can’t migrate guest memory closer to the CPUs running it → performance hit.
> - Memory management: unused guest memory can’t be swapped out, compacted, or merged.
> - Provisioning time: upfront allocation/pinning slows guest create/destroy.
> - Overcommit: no memory overcommit on hosts with pinned-guest memory.
> 
> This series adds movable memory pages for Hyper-V child partitions. Guest
> pages are no longer allocated upfront; they’re allocated and mapped into
> the hypervisor on demand (i.e., when the guest touches a GFN that isn’t yet
> backed by a host PFN).
> When a page is moved, Linux no longer holds it and it is unmapped from the hypervisor.
> As a result, Hyper-V guests behave like regular Linux processes, enabling standard Linux memory features to apply to guests.
> 
> Exceptions (still pinned):
>   1. Encrypted guests (explicit).
>   2 Guests with passthrough devices (implicitly pinned by the VFIO framework).
> 
> v2:
> - Split unmap batching into a separate patch.
> - Fixed commit messages from v1 review.
> - Renamed a few functions for clarity.
> 
> ---
> 
> Stanislav Kinsburskii (4):
>       Drivers: hv: Refactor and rename memory region handling functions
>       Drivers: hv: Centralize guest memory region destruction
>       Drivers: hv: Batch GPA unmap operations to improve large region performance
>       Drivers: hv: Add support for movable memory regions
> 

Our internal test discovered an issue which caused a kernel panic. Has
that been fixed in this series?

Without fixing that bug or ruling out problems in the code I don't think
I can merge this. Once this feature goes into other people's systems it
will be extremely difficult to debug.

Wei


> 
>  drivers/hv/Kconfig             |    1 
>  drivers/hv/mshv_root.h         |   10 +
>  drivers/hv/mshv_root_hv_call.c |    2 
>  drivers/hv/mshv_root_main.c    |  472 +++++++++++++++++++++++++++++++++-------
>  4 files changed, 403 insertions(+), 82 deletions(-)
>