On Wed, Oct 01, 2025 at 04:18:30AM +0000, Wei Liu wrote: > +Mike Rapoport, our resident memory management expert. > > On Fri, Sep 26, 2025 at 07:02:02PM -0700, Mukesh R wrote: > > 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 maybe outdated, and don't have time right now.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. > > > > Mike, I seem to remember you had a discussion with Stanislav about this? > Can you confirm that this is a reasonable approach? > > Better yet, if you have time to review the code, that would be great. > Note that there is a v2 on linux-hyperv. But I would like to close > Mukesh's question first.
I only had time to skip through the patches and yes, this is a reasonable approach. I also confirmed privately with HMM maintainer a while ago that the use of HMM and MMU notifiers is correct. I don't know enough about mshv to see if there are corner cases that these patches don't cover, but conceptually they make memory model follow KVM best practices. > > 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. The usual way we do things in the kernel is to add functionality when it has users, so a boot option can be added later when minkernel design will be more mature and ready for upstream. -- Sincerely yours, Mike.
