Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

John Hubbard
2024-06-19T02:44:57Z

On 6/18/24 5:05 PM, Elliot Berman wrote:
> 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.
>
> [1]: https://lore.kernel.org/all/20240319143119.GA2736@willie-the-truck/
>

Hi!

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).

Maybe a good topic for LPC!

thanks,
--
John Hubbard
NVIDIA

> Tree with patches at:
> https://git.codelinaro.org/clo/linux-kernel/gunyah-linux/-/tree/sent/exclusive-gup-v1
>
> 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,