linux-kernel - Re: [PATCH v4 06/16] x86/virt/tdx: Improve PAMT refcounts allocation for sparse memory

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Message-ID: <468165b7-46aa-4321-a47f-a97befaa993f@linux.intel.com>
Date: Tue, 25 Nov 2025 11:15:22 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: bp@...en8.de, chao.gao@...el.com, dave.hansen@...el.com,
 isaku.yamahata@...el.com, kai.huang@...el.com, kas@...nel.org,
 kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org, mingo@...hat.com, pbonzini@...hat.com,
 seanjc@...gle.com, tglx@...utronix.de, vannapurve@...gle.com,
 x86@...nel.org, yan.y.zhao@...el.com, xiaoyao.li@...el.com,
 binbin.wu@...el.com
Subject: Re: [PATCH v4 06/16] x86/virt/tdx: Improve PAMT refcounts allocation
 for sparse memory



On 11/21/2025 8:51 AM, Rick Edgecombe wrote:
[...]
> +
> +/* 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);
> +	/* refcount allocation is sparse, may not be populated */

Not sure this comment about "sparse" is accurate since this function is called via
apply_to_existing_page_range().

And the check for not present just for sanity check?
> +	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 +393,19 @@ 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;
>   	}
>   
>   	return 0;
>   err:
> +	/*
> +	 * Only free TDX memory blocks here, PAMT refcount pages
> +	 * will be freed in the init_tdx_module() error path.
> +	 */
>   	free_tdx_memlist(tmb_list);
>   	return ret;
>   }