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

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Message-ID: <f1018ab125eb18f431ddb3dd50501914b396ee2b.camel@intel.com>
Date: Fri, 19 Sep 2025 07:25:31 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "kas@...nel.org"
	<kas@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...hat.com"
	<mingo@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "Annapurve, Vishal" <vannapurve@...gle.com>, "Gao,
 Chao" <chao.gao@...el.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>
CC: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters
 allocation for sparse memory


> +/* 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.

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 */
> +	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?