Re: [PATCH 1/5] mm: Add support for unaccepted memory

[David Hildenbrand]

On 10.08.21 08:26, Kirill A. Shutemov wrote:
> UEFI Specification version 2.9 introduces concept of memory acceptance:
> Some Virtual Machine platforms, such as Intel TDX or AMD SEV-SNP,
> requiring memory to be accepted before it can be used by the guest.
> Accepting happens via a protocol specific for the Virtrual Machine
> platform.
>
> Accepting memory is costly and it makes VMM allocate memory for the
> accepted guest physical address range. It's better to postpone memory
> acceptation until memory is needed. It lowers boot time and reduces
> memory overhead.
>
> Support of such memory requires few changes in core-mm code:
>
>    - memblock has to accept memory on allocation;
>
>    - page allocator has to accept memory on the first allocation of the
>      page;
>
> Memblock change is trivial.
>
> Page allocator is modified to accept pages on the first allocation.
> PageOffline() is used to indicate that the page requires acceptance.
> The flag currently used by hotplug and balloon. Such pages are not
> available to page allocator.
>
> An architecture has to provide three helpers if it wants to support
> unaccepted memory:
>
>   - accept_memory() makes a range of physical addresses accepted.
>
>   - maybe_set_page_offline() marks a page PageOffline() if it requires
>     acceptance. Used during boot to put pages on free lists.
>
>   - clear_page_offline() clears makes a page accepted and clears
>     PageOffline().
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>   mm/internal.h   | 14 ++++++++++++++
>   mm/memblock.c   |  1 +
>   mm/page_alloc.c | 13 ++++++++++++-
>   3 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 31ff935b2547..d2fc8a17fbe0 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -662,4 +662,18 @@ void vunmap_range_noflush(unsigned long start, unsigned long end);
>   int numa_migrate_prep(struct page *page, struct vm_area_struct *vma,
>   		      unsigned long addr, int page_nid, int *flags);
>   
> +#ifndef CONFIG_UNACCEPTED_MEMORY
> +static inline void maybe_set_page_offline(struct page *page, unsigned int order)
> +{
> +}
> +
> +static inline void clear_page_offline(struct page *page, unsigned int order)
> +{
> +}
> +
> +static inline void accept_memory(phys_addr_t start, phys_addr_t end)
> +{
> +}

Can we find better fitting names for the first two? The function names 
are way too generic. For example:

accept_or_set_page_offline()

accept_and_clear_page_offline()

I thought for a second if
	PAGE_TYPE_OPS(Unaccepted, offline)
makes sense as well, not sure.


Also, please update the description of PageOffline in page-flags.h to 
include the additional usage with PageBuddy set at the same time.


I assume you don't have to worry about page_offline_freeze/thaw ... as 
we only set PageOffline initially, but not later at runtime when other 
subsystems (/proc/kcore) might stumble over it.

--
Thanks,

David / dhildenb