Re: [PATCH, RFC 46/62] x86/mm: Keep reference counts on encrypted VMAs for MKTME

[Alison Schofield]

On Fri, Jun 14, 2019 at 01:54:24PM +0200, Peter Zijlstra wrote:
> On Wed, May 08, 2019 at 05:44:06PM +0300, Kirill A. Shutemov wrote:
> > From: Alison Schofield <alison.schofield@xxxxxxxxx>
> > 
> > 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);
> >  }

snip

> 
> counts pages, what gives?

Yeah. Comments are confusing. We implemented the refcounting w VMA's in
mind, and then added the page counting. I'll update the comments and
dig around some more based on your overall concerns about the
refcounting you mentioned in the cover letter.