In arm64 pKVM and QuIC's Gunyah protected VM model, we want to support
grabbing shmem user pages instead of using KVM's guestmemfd. These
hypervisors provide a different isolation model than the CoCo
implementations from x86. KVM's guest_memfd is focused on providing
memory that is more isolated than AVF requires. Some specific examples
include ability to pre-load data onto guest-private pages, dynamically
sharing/isolating guest pages without copy, and (future) migrating
guest-private pages. In sum of those differences after a discussion in
[1] and at PUCK, we want to try to stick with existing shmem and extend
GUP to support the isolation needs for arm64 pKVM and Gunyah. To that
end, we introduce the concept of "exclusive GUP pinning", which enforces
that only one pin of any kind is allowed when using the FOLL_EXCLUSIVE
flag is set. This behavior doesn't affect FOLL_GET or any other folio
refcount operations that don't go through the FOLL_PIN path.
Looking through this, I feel that some intangible threshold of "this is
too much overloading of page->_refcount" has been crossed. This is a very
specific feature, and it is using approximately one more bit than is
really actually "available"...
If we need a bit in struct page/folio, is this really the only way? Willy
is working towards getting us an entirely separate folio->pincount, I
suppose that might take too long? Or not?
This feels like force-fitting a very specific feature (KVM/CoCo handling
of shmem pages) into a more general mechanism that is running low on
bits (gup/pup).
--
John Hubbard
NVIDIA
> Tree with patches at:
>
https://git.codelinaro.org/clo/linux-kernel/gunyah-linux/-/tree/sent/exclusi...
>
> anup@brainfault.org, paul.walmsley@sifive.com,
> palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com,
> viro@zeniv.linux.org.uk, brauner@kernel.org,
> willy@infradead.org, akpm@linux-foundation.org,
> xiaoyao.li@intel.com, yilun.xu@intel.com,
> chao.p.peng@linux.intel.com, jarkko@kernel.org,
> amoorthy@google.com, dmatlack@google.com,
> yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com,
> mic@digikod.net, vbabka@suse.cz, vannapurve@google.com,
> ackerleytng@google.com, mail@maciej.szmigiero.name,
> david@redhat.com, michael.roth@amd.com, wei.w.wang@intel.com,
> liam.merwick@oracle.com, isaku.yamahata@gmail.com,
> kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com,
> steven.price@arm.com, quic_eberman@quicinc.com,
> quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com,
> quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com,
> quic_pderrin@quicinc.com, quic_pheragu@quicinc.com,
> catalin.marinas@arm.com, james.morse@arm.com,
> yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org,
> will@kernel.org, qperret@google.com, keirf@google.com,
> tabba@google.com
>
> Signed-off-by: Elliot Berman
quic_eberman@quicinc.com
> ---
> Elliot Berman (2):
> mm/gup-test: Verify exclusive pinned
> mm/gup_test: Verify GUP grabs same pages twice
>
> Fuad Tabba (3):
> mm/gup: Move GUP_PIN_COUNTING_BIAS to page_ref.h
> mm/gup: Add an option for obtaining an exclusive pin
> mm/gup: Add support for re-pinning a normal pinned page as exclusive
>
> include/linux/mm.h | 57 ++++----
> include/linux/mm_types.h | 2 +
> include/linux/page_ref.h | 74 ++++++++++
> mm/Kconfig | 5 +
> mm/gup.c | 265 ++++++++++++++++++++++++++++++----
> mm/gup_test.c | 108 ++++++++++++++
> mm/gup_test.h | 1 +
> tools/testing/selftests/mm/gup_test.c | 5 +-
> 8 files changed, 457 insertions(+), 60 deletions(-)
> ---
> base-commit: 6ba59ff4227927d3a8530fc2973b80e94b54d58f
> change-id: 20240509-exclusive-gup-66259138bbff
>
> Best regards,
