Re: [PATCH v5 0/2] mm: Refactor KVM guest_memfd to introduce guestmem library

[Mike Day <michael.day@amd.com>]

On 11/22/24 11:29, Elliot Berman wrote:
> In preparation for adding more features to KVM's guest_memfd, refactor
> and introduce a library which abtracts some of the core-mm decisions
> about managing folios associated with guest memory. The goal of the
> refactor serves two purposes:
> 
> 1. Provide an easier way to reason about memory in guest_memfd. KVM
>     needs to support multiple confidentiality models (TDX, SEV, pKVM, Arm
>     CCA). These models support different semantics for when the host
>     can(not) access guest memory. An abstraction for the allocator and
>     managing the state of pages will make it eaiser to reason about the
>     state of folios within the guest_memfd.
> 
> 2. Provide a common implementation for other users such as Gunyah [1] and
>     guestmemfs [2].
> 
> In this initial series, I'm seeking comments for the line I'm drawing
> between library and user (KVM). I've not introduced new functionality in
> this series; the first new feature will probably be Fuad's mappability
> patches [3].
> 
> I've decided to only bring out the address_space from guest_memfd as it
> seemed the simplest approach. In the current iteration, KVM "attaches"
> the guestmem to the inode. I expect we'll want to provide some helpers
> for inode, file, and vm operations when it's relevant to
> mappability/accessiblity/faultability.
> 
> I'd appreciate any feedback, especially on how much we should pull into
> the guestmem library.
> 
> [1]: https://lore.kernel.org/lkml/20240222-gunyah-v17-0-1e9da6763d38@quicinc.com/
> [2]: https://lore.kernel.org/all/20240805093245.889357-1-jgowans@amazon.com/
> [3]: https://lore.kernel.org/all/20241010085930.1546800-3-tabba@google.com/
> 
> Changes in v5:
> - Free all folios when the owner removes detaches the guestmem
> - Link to v4: https://lore.kernel.org/r/20241120-guestmem-library-v4-0-0c597f733909@quicinc.com
> 
> Changes in v4:
> - Update folio_free() to add address_space mapping instead of
>    invalidate_folio/free_folio path.
> - Link to v3: https://lore.kernel.org/r/20241113-guestmem-library-v3-0-71fdee85676b@quicinc.com
> 
> Changes in v3:
>   - Refactor/extract only the address_space
>   - Link to v2: https://lore.kernel.org/all/20240829-guest-memfd-lib-v2-0-b9afc1ff3656@quicinc.com/
> 
> Changes in v2:
> - Significantly reworked to introduce "accessible" and "safe" reference
>    counters
> - Link to v1: https://lore.kernel.org/r/20240805-guest-memfd-lib-v1-0-e5a29a4ff5d7@quicinc.com
> 
> Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>

Tested-by: Mike Day <michael.day@amd.com>

> ---
> Elliot Berman (2):
>        filemap: Pass address_space mapping to ->free_folio()
>        mm: guestmem: Convert address_space operations to guestmem library
> 
>   Documentation/filesystems/locking.rst |   2 +-
>   MAINTAINERS                           |   2 +
>   fs/nfs/dir.c                          |  11 +-
>   fs/orangefs/inode.c                   |   3 +-
>   include/linux/fs.h                    |   2 +-
>   include/linux/guestmem.h              |  34 ++++++
>   mm/Kconfig                            |   3 +
>   mm/Makefile                           |   1 +
>   mm/filemap.c                          |   9 +-
>   mm/guestmem.c                         | 201 ++++++++++++++++++++++++++++++++++
>   mm/secretmem.c                        |   3 +-
>   mm/vmscan.c                           |   4 +-
>   virt/kvm/Kconfig                      |   1 +
>   virt/kvm/guest_memfd.c                |  98 +++++------------
>   14 files changed, 290 insertions(+), 84 deletions(-)
> ---
> base-commit: 5cb1659f412041e4780f2e8ee49b2e03728a2ba6
> change-id: 20241112-guestmem-library-68363cb29186
> 
> Best regards,