Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

On 6/14/2022 2:02 PM, Kirill A. Shutemov wrote:
> UEFI Specification version 2.9 introduces the concept of memory
> acceptance. Some Virtual Machine platforms, such as Intel TDX or AMD
> SEV-SNP, require memory to be accepted before it can be used by the
> guest. Accepting happens via a protocol specific to the Virtual Machine
> platform.
> 
> There are several ways kernel can deal with unaccepted memory:
> 
>   1. Accept all the memory during the boot. It is easy to implement and
>      it doesn't have runtime cost once the system is booted. The downside
>      is very long boot time.
> 
>      Accept can be parallelized to multiple CPUs to keep it manageable
>      (i.e. via DEFERRED_STRUCT_PAGE_INIT), but it tends to saturate
>      memory bandwidth and does not scale beyond the point.
> 
>   2. Accept a block of memory on the first use. It requires more
>      infrastructure and changes in page allocator to make it work, but
>      it provides good boot time.
> 
>      On-demand memory accept means latency spikes every time kernel steps
>      onto a new memory block. The spikes will go away once workload data
>      set size gets stabilized or all memory gets accepted.
> 
>   3. Accept all memory in background. Introduce a thread (or multiple)
>      that gets memory accepted proactively. It will minimize time the
>      system experience latency spikes on memory allocation while keeping
>      low boot time.
> 
>      This approach cannot function on its own. It is an extension of #2:
>      background memory acceptance requires functional scheduler, but the
>      page allocator may need to tap into unaccepted memory before that.
> 
>      The downside of the approach is that these threads also steal CPU
>      cycles and memory bandwidth from the user's workload and may hurt
>      user experience.
> 
> Implement #2 for now. It is a reasonable default. Some workloads may
> want to use #1 or #3 and they can be implemented later based on user's
> demands.
> 
> Support of unaccepted memory requires a 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.
> 
> The page allocator is modified to accept pages on the first allocation.
> The new page type (encoded in the _mapcount) -- PageUnaccepted() -- is
> used to indicate that the page requires acceptance.
> 
> Architecture has to provide two helpers if it wants to support
> unaccepted memory:
> 
>   - accept_memory() makes a range of physical addresses accepted.
> 
>   - range_contains_unaccepted_memory() checks anything within the range
>     of physical addresses requires acceptance.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: Mike Rapoport <rppt@linux.ibm.com>	# memblock
> Reviewed-by: David Hildenbrand <david@redhat.com>
> ---
>   include/linux/page-flags.h | 31 +++++++++++++
>   mm/internal.h              | 12 +++++
>   mm/memblock.c              |  9 ++++
>   mm/page_alloc.c            | 89 +++++++++++++++++++++++++++++++++++++-
>   4 files changed, 139 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index e66f7aa3191d..444ba8f4bfa0 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -942,6 +942,14 @@ static inline bool is_page_hwpoison(struct page *page)
>   #define PG_offline	0x00000100
>   #define PG_table	0x00000200
>   #define PG_guard	0x00000400
> +#define PG_unaccepted	0x00000800
> +
> +/*
> + * Page types allowed at page_expected_state()
> + *
> + * PageUnaccepted() will get cleared in post_alloc_hook().
> + */
> +#define PAGE_TYPES_EXPECTED	PG_unaccepted
>   
>   #define PageType(page, flag)						\
>   	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> @@ -967,6 +975,18 @@ static __always_inline void __ClearPage##uname(struct page *page)	\
>   	page->page_type |= PG_##lname;					\
>   }
>   
> +#define PAGE_TYPE_OPS_FALSE(uname)					\
> +static __always_inline int Page##uname(struct page *page)		\
> +{									\
> +	return false;							\
> +}									\
> +static __always_inline void __SetPage##uname(struct page *page)		\
> +{									\
> +}									\
> +static __always_inline void __ClearPage##uname(struct page *page)	\
> +{									\
> +}
> +
>   /*
>    * PageBuddy() indicates that the page is free and in the buddy system
>    * (see mm/page_alloc.c).
> @@ -997,6 +1017,17 @@ PAGE_TYPE_OPS(Buddy, buddy)
>    */
>   PAGE_TYPE_OPS(Offline, offline)
>   
> +/*
> + * PageUnaccepted() indicates that the page has to be "accepted" before it can
> + * be read or written. The page allocator must call accept_page() before
> + * touching the page or returning it to the caller.
> + */
> +#ifdef CONFIG_UNACCEPTED_MEMORY
> +PAGE_TYPE_OPS(Unaccepted, unaccepted)
> +#else
> +PAGE_TYPE_OPS_FALSE(Unaccepted)
> +#endif
> +
>   extern void page_offline_freeze(void);
>   extern void page_offline_thaw(void);
>   extern void page_offline_begin(void);
> diff --git a/mm/internal.h b/mm/internal.h
> index c0f8fbe0445b..7c1a653edfc8 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -861,4 +861,16 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags);
>   
>   DECLARE_PER_CPU(struct per_cpu_nodestat, boot_nodestats);
>   
> +#ifndef CONFIG_UNACCEPTED_MEMORY
> +static inline bool range_contains_unaccepted_memory(phys_addr_t start,
> +						    phys_addr_t end)
> +{
> +	return false;
> +}
> +
> +static inline void accept_memory(phys_addr_t start, phys_addr_t end)
> +{
> +}
> +#endif
> +
>   #endif	/* __MM_INTERNAL_H */
> diff --git a/mm/memblock.c b/mm/memblock.c
> index e4f03a6e8e56..a1f7f8b304d5 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1405,6 +1405,15 @@ phys_addr_t __init memblock_alloc_range_nid(phys_addr_t size,
>   		 */
>   		kmemleak_alloc_phys(found, size, 0, 0);
>   
> +	/*
> +	 * Some Virtual Machine platforms, such as Intel TDX or AMD SEV-SNP,
> +	 * require memory to be accepted before it can be used by the
> +	 * guest.
> +	 *
> +	 * Accept the memory of the allocated buffer.
> +	 */
> +	accept_memory(found, found + size);
> +
>   	return found;
>   }
>   
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e008a3df0485..279c2746aaa8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -122,6 +122,12 @@ typedef int __bitwise fpi_t;
>    */
>   #define FPI_SKIP_KASAN_POISON	((__force fpi_t)BIT(2))
>   
> +/*
> + * Check if the page needs to be marked as PageUnaccepted().
> + * Used for the new pages added to the buddy allocator for the first time.
> + */
> +#define FPI_UNACCEPTED_SLOWPATH	((__force fpi_t)BIT(3))
> +
>   /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
>   static DEFINE_MUTEX(pcp_batch_high_lock);
>   #define MIN_PERCPU_PAGELIST_HIGH_FRACTION (8)
> @@ -993,6 +999,35 @@ buddy_merge_likely(unsigned long pfn, unsigned long buddy_pfn,
>   			NULL) != NULL;
>   }
>   
> +/*
> + * Page acceptance can be very slow. Do not call under critical locks.
> + */
> +static void accept_page(struct page *page, unsigned int order)
> +{
> +	phys_addr_t start = page_to_phys(page);
> +	int i;
> +
> +	if (!PageUnaccepted(page))
> +		return;
> +
> +	accept_memory(start, start + (PAGE_SIZE << order));
> +	__ClearPageUnaccepted(page);
> +
> +	/* Assert that there is no PageUnaccepted() on tail pages */
> +	if (IS_ENABLED(CONFIG_DEBUG_VM)) {
> +		for (i = 1; i < (1 << order); i++)
> +			VM_BUG_ON_PAGE(PageUnaccepted(page + i), page + i);
> +	}
> +}
> +
> +static bool page_contains_unaccepted(struct page *page, unsigned int order)
> +{
> +	phys_addr_t start = page_to_phys(page);
> +	phys_addr_t end = start + (PAGE_SIZE << order);
> +
> +	return range_contains_unaccepted_memory(start, end);
> +}
> +
>   /*
>    * Freeing function for a buddy system allocator.
>    *
> @@ -1027,6 +1062,7 @@ static inline void __free_one_page(struct page *page,
>   	unsigned long combined_pfn;
>   	struct page *buddy;
>   	bool to_tail;
> +	bool page_needs_acceptance = false;
>   
>   	VM_BUG_ON(!zone_is_initialized(zone));
>   	VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
> @@ -1038,6 +1074,11 @@ static inline void __free_one_page(struct page *page,
>   	VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
>   	VM_BUG_ON_PAGE(bad_range(zone, page), page);
>   
> +	if (PageUnaccepted(page)) {
> +		page_needs_acceptance = true;
> +		__ClearPageUnaccepted(page);
> +	}
> +
>   	while (order < MAX_ORDER - 1) {
>   		if (compaction_capture(capc, page, order, migratetype)) {
>   			__mod_zone_freepage_state(zone, -(1 << order),
> @@ -1072,6 +1113,13 @@ static inline void __free_one_page(struct page *page,
>   			clear_page_guard(zone, buddy, order, migratetype);
>   		else
>   			del_page_from_free_list(buddy, zone, order);
> +
> +		/* Mark page unaccepted if any of merged pages were unaccepted */
> +		if (PageUnaccepted(buddy)) {
> +			page_needs_acceptance = true;
> +			__ClearPageUnaccepted(buddy);
> +		}
> +
>   		combined_pfn = buddy_pfn & pfn;
>   		page = page + (combined_pfn - pfn);
>   		pfn = combined_pfn;
> @@ -1081,6 +1129,23 @@ static inline void __free_one_page(struct page *page,
>   done_merging:
>   	set_buddy_order(page, order);
>   
> +	/*
> +	 * The page gets marked as PageUnaccepted() if any of merged-in pages
> +	 * is PageUnaccepted().
> +	 *
> +	 * New pages, just being added to buddy allocator, do not have
> +	 * PageUnaccepted() set. FPI_UNACCEPTED_SLOWPATH indicates that the
> +	 * page is new and page_contains_unaccepted() check is required to
> +	 * determinate if acceptance is required.
> +	 *
> +	 * Avoid calling page_contains_unaccepted() if it is known that the page
> +	 * needs acceptance. It can be costly.
> +	 */
> +	if (!page_needs_acceptance && (fpi_flags & FPI_UNACCEPTED_SLOWPATH))
> +		page_needs_acceptance = page_contains_unaccepted(page, order);
> +	if (page_needs_acceptance)
> +		__SetPageUnaccepted(page);
> +
>   	if (fpi_flags & FPI_TO_TAIL)
>   		to_tail = true;
>   	else if (is_shuffle_order(order))
> @@ -1164,7 +1229,13 @@ int split_free_page(struct page *free_page,
>   static inline bool page_expected_state(struct page *page,
>   					unsigned long check_flags)
>   {
> -	if (unlikely(atomic_read(&page->_mapcount) != -1))
> +	/*
> +	 * The page must not be mapped to userspace and must not have
> +	 * a PageType other than listed in PAGE_TYPES_EXPECTED.
> +	 *
> +	 * Note, bit cleared means the page type is set.
> +	 */
> +	if (unlikely((atomic_read(&page->_mapcount) | PAGE_TYPES_EXPECTED) != -1))
>   		return false;
>   
>   	if (unlikely((unsigned long)page->mapping |
> @@ -1669,7 +1740,9 @@ void __free_pages_core(struct page *page, unsigned int order)
>   	 * Bypass PCP and place fresh pages right to the tail, primarily
>   	 * relevant for memory onlining.
>   	 */
> -	__free_pages_ok(page, order, FPI_TO_TAIL | FPI_SKIP_KASAN_POISON);
> +	__free_pages_ok(page, order,
> +			FPI_TO_TAIL | FPI_SKIP_KASAN_POISON |
> +			FPI_UNACCEPTED_SLOWPATH);
>   }
>   
>   #ifdef CONFIG_NUMA
> @@ -1822,6 +1895,9 @@ static void __init deferred_free_range(unsigned long pfn,
>   		return;
>   	}
>   
> +	/* Accept chunks smaller than page-block upfront */
> +	accept_memory(PFN_PHYS(pfn), PFN_PHYS(pfn + nr_pages));
> +
>   	for (i = 0; i < nr_pages; i++, page++, pfn++) {
>   		if ((pfn & (pageblock_nr_pages - 1)) == 0)
>   			set_pageblock_migratetype(page, MIGRATE_MOVABLE);
> @@ -2281,6 +2357,13 @@ static inline void expand(struct zone *zone, struct page *page,
>   		if (set_page_guard(zone, &page[size], high, migratetype))
>   			continue;
>   
> +		/*
> +		 * Transfer PageUnaccepted() to the newly split pages so
> +		 * they can be accepted after dropping the zone lock.
> +		 */
> +		if (PageUnaccepted(page))
> +			__SetPageUnaccepted(&page[size]);
> +
>   		add_to_free_list(&page[size], zone, high, migratetype);
>   		set_buddy_order(&page[size], high);
>   	}
> @@ -2411,6 +2494,8 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>   	 */
>   	kernel_unpoison_pages(page, 1 << order);
>   
> +	accept_page(page, order);
> +
>   	/*
>   	 * As memory initialization might be integrated into KASAN,
>   	 * KASAN unpoisoning and memory initializion code must be

Reviewed previous version(v6) of this patch. Seems no change in this 
version except '!PageUnaccepted' check addition in function 
'accept_page' and rename of function 'page_contains_unaccepted'.

Acked-by: Pankaj Gupta <pankaj.gupta@amd.com>