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

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Date: Tue, 18 Jun 2024 19:44:44 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Elliot Berman <quic_eberman@...cinc.com>, Andrew Morton
	<akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>, David Hildenbrand
	<david@...hat.com>, Matthew Wilcox <willy@...radead.org>, <maz@...nel.org>
CC: <kvm@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>, <pbonzini@...hat.com>, Fuad Tabba
	<tabba@...gle.com>
Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

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@...infault.org, paul.walmsley@...ive.com,
> 	palmer@...belt.com,  aou@...s.berkeley.edu, seanjc@...gle.com,
> 	viro@...iv.linux.org.uk,  brauner@...nel.org,
> 	willy@...radead.org, akpm@...ux-foundation.org,
> 	 xiaoyao.li@...el.com, yilun.xu@...el.com,
> 	chao.p.peng@...ux.intel.com,  jarkko@...nel.org,
> 	amoorthy@...gle.com, dmatlack@...gle.com,
> 	 yu.c.zhang@...ux.intel.com, isaku.yamahata@...el.com,
> 	mic@...ikod.net,  vbabka@...e.cz, vannapurve@...gle.com,
> 	ackerleytng@...gle.com,  mail@...iej.szmigiero.name,
> 	david@...hat.com, michael.roth@....com,  wei.w.wang@...el.com,
> 	liam.merwick@...cle.com, isaku.yamahata@...il.com,
> 	 kirill.shutemov@...ux.intel.com, suzuki.poulose@....com,
> 	steven.price@....com,  quic_eberman@...cinc.com,
> 	quic_mnalajal@...cinc.com, quic_tsoni@...cinc.com,
> 	 quic_svaddagi@...cinc.com, quic_cvanscha@...cinc.com,
> 	 quic_pderrin@...cinc.com, quic_pheragu@...cinc.com,
> 	catalin.marinas@....com,  james.morse@....com,
> 	yuzenghui@...wei.com, oliver.upton@...ux.dev,  maz@...nel.org,
> 	will@...nel.org, qperret@...gle.com, keirf@...gle.com,
> 	 tabba@...gle.com
> 
> Signed-off-by: Elliot Berman <quic_eberman@...cinc.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,