Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

Kirill A. Shutemov
2022-04-09T15:52:56Z

On Fri, Apr 08, 2022 at 11:55:43AM -0700, Dave Hansen wrote:
> On 4/5/22 16:43, 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, requiring memory to be accepted before it can be used by the
>
> ^ require

Heh. That's wording form the spec.

> > guest. Accepting happens via a protocol specific for the Virtual Machine
> > platform.
>
> ^ s/for/to
>
> > Accepting memory is costly and it makes VMM allocate memory for the
> > accepted guest physical address range. It's better to postpone memory
> > acceptance until memory is needed. It lowers boot time and reduces
> > memory overhead.
> >
> > Support of such 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.
> > PageUnaccepted() is used to indicate that the page requires acceptance.
>
> Does this consume an actual page flag or is it aliased?

It is encoded as a page type in mapcount of unallocated memory. It is not
aliased with PageOffline() as I did before.

I will mention that it is a new page type.

> > Kernel only needs to accept memory once after boot, so during the boot
> > and warm up phase there will be a lot of memory acceptance. After things
> > are settled down the only price of the feature if couple of checks for
> > PageUnaccepted() in allocate and free paths. The check refers a hot
>
> ^ to
>
> ...
> > + /*
> > + * PageUnaccepted() indicates that the page has to be "accepted" before it can
> > + * be used. Page allocator has to call accept_page() before returning the page
> > + * to the caller.
> > + */
>
> Let's talk about "used" with a bit more detail. Maybe:
>
> /*
> * PageUnaccepted() indicates that the page has to be "accepted" before
> * it can be read or written. The page allocator must to call
> * accept_page() before touching the page or returning it to the caller.
> */

I guess s/must to call/must call/, right?

> ...
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 2db95780e003..53f4aa1c92a7 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -121,6 +121,12 @@ 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 ((__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)
> > @@ -1023,6 +1029,26 @@ buddy_merge_likely(unsigned long pfn, unsigned long buddy_pfn,
> > return page_is_buddy(higher_page, higher_buddy, order + 1);
> > }
> >
> > +static void accept_page(struct page *page, unsigned int order)
> > +{
> > + phys_addr_t start = page_to_phys(page);
> > + int i;
> > +
> > + accept_memory(start, start + (PAGE_SIZE << order));
> > +
> > + for (i = 0; i < (1 << order); i++) {
> > + if (PageUnaccepted(page + i))
> > + __ClearPageUnaccepted(page + i);
> > + }
> > +}
>
> It's probably worth a comment somewhere that this can be really slow.
>
> > +static bool page_is_unaccepted(struct page *page, unsigned int order)
> > +{
> > + phys_addr_t start = page_to_phys(page);
> > +
> > + return memory_is_unaccepted(start, start + (PAGE_SIZE << order));
> > +}
> > +
> > /*
> > * Freeing function for a buddy system allocator.
> > *
> > @@ -1058,6 +1084,7 @@ static inline void __free_one_page(struct page *page,
> > unsigned long combined_pfn;
> > struct page *buddy;
> > bool to_tail;
> > + bool unaccepted = PageUnaccepted(page);
> >
> > VM_BUG_ON(!zone_is_initialized(zone));
> > VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
> > @@ -1089,6 +1116,11 @@ 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))
> > + unaccepted = true;
>
> Naming nit: following the logic with a double-negative like !unaccepted
> is a bit hard. Would this be more readable if it were:
>
> bool page_needs_acceptance = PageUnaccepted(page);
>
> and then the code below...
>
> > combined_pfn = buddy_pfn & pfn;
> > page = page + (combined_pfn - pfn);
> > pfn = combined_pfn;
> > @@ -1124,6 +1156,17 @@ static inline void __free_one_page(struct page *page,
> > done_merging:
> > set_buddy_order(page, order);
> >
> > + /*
> > + * Check if the page needs to be marked as PageUnaccepted().
> > + * Used for the new pages added to the buddy allocator for the first
> > + * time.
> > + */
> > + if (!unaccepted && (fpi_flags & FPI_UNACCEPTED))
> > + unaccepted = page_is_unaccepted(page, order);
>
> if (page_needs_acceptance && (fpi_flags & FPI_UNACCEPTED))
> page_needs_acceptance = page_is_unaccepted(page, order);
>
> > + if (unaccepted)
> > + __SetPageUnaccepted(page);
>
> This is getting hard for me to follow.
>
> There are:
> 1. Pages that come in here with PageUnaccepted()==1
> 2. Pages that come in here with PageUnaccepted()==0, but a buddy that
> was PageUnaccepted()==1
>
> In either of those cases, the bitmap will be consulted to see if the
> page is *truly* unaccepted or not. But, I'm struggling to figure out
> how a page could end up in one of those scenarios and *not* be
> page_is_unaccepted().
>
> There are three pieces of information that come in:
> 1. PageUnaccepted(page)
> 2. PageUnaccepted(buddies[])
> 3. the bitmap

1 and 2 are the same conceptionally: merged-in pieces of the resulting page.

>
> and one piece of information going out:
>
> PageUnaccepted(page);
>
> I think I need a more coherent description of how those four things fit
> together.

The page gets marked as PageUnaccepted() if any of merged-in pages is
PageUnaccepted().

For new pages, just being added to buddy allocator, consult
page_is_unaccepted(). FPI_UNACCEPTED indicates that the page is new and
page_is_unaccepted() check is required.

Avoid calling page_is_unaccepted() if it is known that the page needs
acceptance anyway. It can be costly.

Is it good enough explanation?

FPI_UNACCEPTED is not a good name. Any help with a better one?
FPI_CHECK_UNACCEPTED?

> > if (fpi_flags & FPI_TO_TAIL)
> > to_tail = true;
> > else if (is_shuffle_order(order))
> > @@ -1149,7 +1192,8 @@ static inline void __free_one_page(struct page *page,
> > static inline bool page_expected_state(struct page *page,
> > unsigned long check_flags)
> > {
> > - if (unlikely(atomic_read(&page->_mapcount) != -1))
> > + if (unlikely(atomic_read(&page->_mapcount) != -1) &&
> > + !PageUnaccepted(page))
> > return false;
>
> That probably deserves a comment, and maybe its own if() statement.

Own if does not work. PageUnaccepted() is encoded in _mapcount.

What about this:

/*
* page->_mapcount is expected to be -1.
*
* There is an exception for PageUnaccepted(). The page type can be set
* for pages on free list. Page types are encoded in _mapcount.
*
* PageUnaccepted() will get cleared in post_alloc_hook().
*/
if (unlikely((atomic_read(&page->_mapcount) | PG_unaccepted) != -1))
return false;

?

> > if (unlikely((unsigned long)page->mapping |
> > @@ -1654,7 +1698,8 @@ 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);
> > }
> >
> > #ifdef CONFIG_NUMA
> > @@ -1807,6 +1852,7 @@ static void __init deferred_free_range(unsigned long pfn,
> > return;
> > }
> >
> > + accept_memory(pfn << PAGE_SHIFT, (pfn + nr_pages) << PAGE_SHIFT);
> > for (i = 0; i < nr_pages; i++, page++, pfn++) {
> > if ((pfn & (pageblock_nr_pages - 1)) == 0)
> > set_pageblock_migratetype(page, MIGRATE_MOVABLE);
>
> Comment, please. I assume doing the slow accept up front is OK here
> because this is in the deferred path. But, it would be nice to know for
> sure.

It is acceptance of smaller than page block upfront. I will add a comment.

>
> > @@ -2266,6 +2312,10 @@ 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 */
> > + if (PageUnaccepted(page))
> > + __SetPageUnaccepted(&page[size]);
>
> We don't want to just accept the page here, right? Because we're
> holding the zone lock? Maybe we should mention that:
>
> /*
> * Transfer PageUnaccepted() to the newly split pages so
> * they can be accepted after dropping the zone lock.
> */

Okay.

> > add_to_free_list(&page[size], zone, high, migratetype);
> > set_buddy_order(&page[size], high);
> > }
> > @@ -2396,6 +2446,9 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
> > */
> > kernel_unpoison_pages(page, 1 << order);
> >
> > + if (PageUnaccepted(page))
> > + accept_page(page, order);
> > +
> > /*
> > * As memory initialization might be integrated into KASAN,
> > * KASAN unpoisoning and memory initializion code must be
>
> Is accepted memory guaranteed to be zeroed? Do we want to skip the
> __GFP_ZERO behavior later in this function? Or is that just a silly
> over-optimization?

For TDX, it is true that the memory gets cleared on acceptance, but I
don't we can say the same for any possible implementation.

I would rather leave __GFP_ZERO for peace of mind. Clearing the cache-hot
page for the second time shouldn't be a big deal comparing to acceptance
cost.

--
Kirill A. Shutemov