Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters allocation for sparse memory

[Binbin Wu]

On 9/19/2025 3:25 PM, Huang, Kai wrote:
> > +/* Map a page into the PAMT refcount vmalloc region */
> > +static int pamt_refcount_populate(pte_t *pte, unsigned long addr, void *data)
> > +{
> > +	struct page *page;
> > +	pte_t entry;
> >   
> > -	pamt_refcounts = vmalloc(size);
> > -	if (!pamt_refcounts)
> > +	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> > +	if (!page)
> >   		return -ENOMEM;
> >   
> > +	entry = mk_pte(page, PAGE_KERNEL);
> > +
> > +	spin_lock(&init_mm.page_table_lock);
> > +	/*
> > +	 * PAMT refcount populations can overlap due to rounding of the
> > +	 * start/end pfn.
> [...]
>
> > Make sure another PAMT range didn't already
> > +	 * populate it.
> > +	 */
> Make sure the same range only gets populated once ?
>
> > +	if (pte_none(ptep_get(pte)))
> > +		set_pte_at(&init_mm, addr, pte, entry);
> > +	else
> > +		__free_page(page);
> > +	spin_unlock(&init_mm.page_table_lock);
> > +
> >   	return 0;
> >   }
> >   
> > +/*
> > + * Allocate PAMT reference counters for the given PFN range.
> > + *
> > + * It consumes 2MiB for every 1TiB of physical memory.
> > + */
> > +static int alloc_pamt_refcount(unsigned long start_pfn, unsigned long end_pfn)
> > +{
> > +	unsigned long start, end;
> > +
> > +	start = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(start_pfn));
> > +	end   = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(end_pfn + 1));
> (sorry didn't notice this in last version)
>
> I don't quite follow why we need "end_pfn + 1" instead of just "end_pfn"?
>
> IIUC this could result in an additional 2M range being populated
> unnecessarily when the end_pfn is 2M aligned.

IIUC, this will not happen.
The +1 page will be converted to 4KB, and will be ignored since in
tdx_find_pamt_refcount() the address is divided by 2M.

To handle the address unaligned to 2M, +511 should be used instead of +1?

> And ...
>
> > +	start = round_down(start, PAGE_SIZE);
> > +	end   = round_up(end, PAGE_SIZE);
> > +
> > +	return apply_to_page_range(&init_mm, start, end - start,
> > +				   pamt_refcount_populate, NULL);
> > +}
> > +
> > +/*
> > + * Reserve vmalloc range for PAMT reference counters. It covers all physical
> > + * address space up to max_pfn. It is going to be populated from
> > + * build_tdx_memlist() only for present memory that available for TDX use.
> > + *
> > + * It reserves 2MiB of virtual address space for every 1TiB of physical memory.
> > + */
> > +static int init_pamt_metadata(void)
> > +{
> > +	struct vm_struct *area;
> > +	size_t size;
> > +
> > +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> > +		return 0;
> > +
> > +	size = max_pfn / PTRS_PER_PTE * sizeof(*pamt_refcounts);
> > +	size = round_up(size, PAGE_SIZE);
> > +
> > +	area = get_vm_area(size, VM_SPARSE);
> > +	if (!area)
> > +		return -ENOMEM;
> > +
> > +	pamt_refcounts = area->addr;
> > +	return 0;
> > +}
> > +
> > +/* Unmap a page from the PAMT refcount vmalloc region */
> > +static int pamt_refcount_depopulate(pte_t *pte, unsigned long addr, void *data)
> > +{
> > +	struct page *page;
> > +	pte_t entry;
> > +
> > +	spin_lock(&init_mm.page_table_lock);
> > +
> > +	entry = ptep_get(pte);
> > +	/* refount allocation is sparse, may not be populated */

refount -> refcount


> > +	if (!pte_none(entry)) {
> > +		pte_clear(&init_mm, addr, pte);
> > +		page = pte_page(entry);
> > +		__free_page(page);
> > +	}
> > +
> > +	spin_unlock(&init_mm.page_table_lock);
> > +
> > +	return 0;
> > +}
> > +
> > +/* Unmap all PAMT refcount pages and free vmalloc range */
> >   static void free_pamt_metadata(void)
> >   {
> > +	size_t size;
> > +
> >   	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> >   		return;
> >   
> > +	size = max_pfn / PTRS_PER_PTE * sizeof(*pamt_refcounts);
> > +	size = round_up(size, PAGE_SIZE);
> > +
> > +	apply_to_existing_page_range(&init_mm,
> > +				     (unsigned long)pamt_refcounts,
> > +				     size, pamt_refcount_depopulate,
> > +				     NULL);
> >   	vfree(pamt_refcounts);
> >   	pamt_refcounts = NULL;
> >   }
> > @@ -288,10 +377,19 @@ static int build_tdx_memlist(struct list_head *tmb_list)
> >   		ret = add_tdx_memblock(tmb_list, start_pfn, end_pfn, nid);
> >   		if (ret)
> >   			goto err;
> > +
> > +		/* Allocated PAMT refcountes for the memblock */
> > +		ret = alloc_pamt_refcount(start_pfn, end_pfn);
> > +		if (ret)
> > +			goto err;
> >   	}
> ... when max_pfn == end_pfn of the last TDX memory block, this could
> result in an additional page of @pamt_refcounts being allocated, but it
> will never be freed since free_pamt_metadata() will only free mapping up
> to max_pfn.
>
> Am I missing anything?