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

Huang, Kai
2025-09-19T07:25:37Z

> +/* 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?