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SubjectRe: [PATCH, RFC 46/62] x86/mm: Keep reference counts on encrypted VMAs for MKTME
On Wed, May 08, 2019 at 05:44:06PM +0300, Kirill A. Shutemov wrote:
> From: Alison Schofield <alison.schofield@intel.com>
>
> The MKTME (Multi-Key Total Memory Encryption) Key Service needs
> a reference count on encrypted VMAs. This reference count is used
> to determine when a hardware encryption KeyID is no longer in use
> and can be freed and reassigned to another Userspace Key.
>
> The MKTME Key service does the percpu_ref_init and _kill, so
> these gets/puts on encrypted VMA's can be considered the
> intermediaries in the lifetime of the key.
>
> Increment/decrement the reference count during encrypt_mprotect()
> system call for initial or updated encryption on a VMA.
>
> Piggy back on the vm_area_dup/free() helpers. If the VMAs being
> duplicated, or freed are encrypted, adjust the reference count.

That all talks about VMAs, but...

> @@ -102,6 +115,22 @@ void __prep_encrypted_page(struct page *page, int order, int keyid, bool zero)
>
> page++;
> }
> +
> + /*
> + * Make sure the KeyID cannot be freed until the last page that
> + * uses the KeyID is gone.
> + *
> + * This is required because the page may live longer than VMA it
> + * is mapped into (i.e. in get_user_pages() case) and having
> + * refcounting per-VMA is not enough.
> + *
> + * Taking a reference per-4K helps in case if the page will be
> + * split after the allocation. free_encrypted_page() will balance
> + * out the refcount even if the page was split and freed as bunch
> + * of 4K pages.
> + */
> +
> + percpu_ref_get_many(&encrypt_count[keyid], 1 << order);
> }
>
> /*
> @@ -110,7 +139,9 @@ void __prep_encrypted_page(struct page *page, int order, int keyid, bool zero)
> */
> void free_encrypted_page(struct page *page, int order)
> {
> - int i;
> + int i, keyid;
> +
> + keyid = page_keyid(page);
>
> /*
> * The hardware/CPU does not enforce coherency between mappings
> @@ -125,6 +156,8 @@ void free_encrypted_page(struct page *page, int order)
> lookup_page_ext(page)->keyid = 0;
> page++;
> }
> +
> + percpu_ref_put_many(&encrypt_count[keyid], 1 << order);
> }

counts pages, what gives?

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