linux-kernel - Re: [PATCH 0/3] Introduce movable pages for Hyper-V guests

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Message-ID: <0ef4d844-a0af-9fa6-2163-b83f80bc74b1@linux.microsoft.com>
Date: Fri, 26 Sep 2025 19:02:02 -0700
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>,
 kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
 decui@...rosoft.com
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Introduce movable pages for Hyper-V guests

On 9/24/25 14:30, 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).


As I had commented internally, I am not fully comfortable about the
approach here, specially around use of HMM, and the correctness of
locking for shared memory regions, but my knowledge is from 4.15 and
maybe outdated, and don't have time right now. So I won't object to it
if other hard core mmu developers think there are no issues.

However, we won't be using this for minkernel, so would like a driver
boot option to disable it upon boot that we can just set in minkernel
init path. This option can also be used to disable it if problems are
observed on the field. Minkernel design is still being worked on, so I
cannot provide much details on it yet.

Thanks,
-Mukesh


> ---
> 
> Stanislav Kinsburskii (3):
>       Drivers: hv: Rename a few memory region related functions for clarity
>       Drivers: hv: Centralize guest memory region destruction in helper
>       Drivers: hv: Add support for movable memory regions
> 
> 
>  drivers/hv/Kconfig          |    1 
>  drivers/hv/mshv_root.h      |    8 +
>  drivers/hv/mshv_root_main.c |  448 +++++++++++++++++++++++++++++++++++++------
>  3 files changed, 397 insertions(+), 60 deletions(-)
>