Re: [PATCH, RFC 13/62] x86/mm: Add hooks to allocate and free encrypted pages

[Kirill A. Shutemov]

On Fri, Jun 14, 2019 at 01:04:58PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 14, 2019 at 11:34:09AM +0200, Peter Zijlstra wrote:
> > On Wed, May 08, 2019 at 05:43:33PM +0300, Kirill A. Shutemov wrote:
> > 
> > > +		lookup_page_ext(page)->keyid = keyid;
> 
> > > +		lookup_page_ext(page)->keyid = 0;
> 
> Also, perhaps paranoid; but do we want something like:
> 
> static inline void page_set_keyid(struct page *page, int keyid)
> {
> 	/* ensure nothing creeps after changing the keyid */
> 	barrier();
> 	WRITE_ONCE(lookup_page_ext(page)->keyid, keyid);
> 	barrier();
> 	/* ensure nothing creeps before changing the keyid */
> }
> 
> And this is very much assuming there is no concurrency through the
> allocator locks.

There's no concurrency for this page: it has been off the free list, but
have not yet passed on to user. Nobody else sees the page before
allocation is finished.

And barriers/WRITE_ONCE() looks excessive to me. It's just yet another bit
of page's metadata and I don't see why it's has to be handled in a special
way.

Does it relax your paranoia? :P

-- 
 Kirill A. Shutemov