linux-kernel - Re: [RFC PATCH v2 29/51] mm: guestmem_hugetlb: Wrap HugeTLB as an allocator for guest

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Message-ID: <diqzzffcfy3k.fsf@ackerleytng-ctop.c.googlers.com>
Date: Fri, 16 May 2025 07:07:27 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH v2 29/51] mm: guestmem_hugetlb: Wrap HugeTLB as an
 allocator for guest_memfd

Ackerley Tng <ackerleytng@...gle.com> writes:

> guestmem_hugetlb is an allocator for guest_memfd. It wraps HugeTLB to
> provide huge folios for guest_memfd.
>
> This patch also introduces guestmem_allocator_operations as a set of
> operations that allocators for guest_memfd can provide. In a later
> patch, guest_memfd will use these operations to manage pages from an
> allocator.
>
> The allocator operations are memory-management specific and are placed
> in mm/ so key mm-specific functions do not have to be exposed
> unnecessarily.
>
> Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
>
> Change-Id: I3cafe111ea7b3c84755d7112ff8f8c541c11136d
> ---
>  include/linux/guestmem.h      |  20 +++++
>  include/uapi/linux/guestmem.h |  29 +++++++
>  mm/Kconfig                    |   5 +-
>  mm/guestmem_hugetlb.c         | 159 ++++++++++++++++++++++++++++++++++
>  4 files changed, 212 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/guestmem.h
>  create mode 100644 include/uapi/linux/guestmem.h
>
> <snip>
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 131adc49f58d..bb6e39e37245 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1218,7 +1218,10 @@ config SECRETMEM
>  
>  config GUESTMEM_HUGETLB
>  	bool "Enable guestmem_hugetlb allocator for guest_memfd"
> -	depends on HUGETLBFS
> +	select GUESTMEM
> +	select HUGETLBFS
> +	select HUGETLB_PAGE
> +	select HUGETLB_PAGE_OPTIMIZE_VMEMMAP

My bad. I left out CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON=y in
my testing and just found that when it is set, I hit

  BUG_ON(pte_page(ptep_get(pte)) != walk->reuse_page);

with the basic guest_memfd_test on splitting pages on allocation.

I'll follow up with the fix soon.

Another note about testing: I've been testing in a nested VM for the
development process:

1. Host
2. VM for development
3. Nested VM running kernel being developed
4. Nested nested VMs created during selftests

This series has not yet been tested on a physical host.

>  	help
>  	  Enable this to make HugeTLB folios available to guest_memfd
>  	  (KVM virtualization) as backing memory.
>
> <snip>
>