Re: [PATCH] secretmem: optimize page_is_secretmem()

[Mike Rapoport <rppt@kernel.org>]

On Mon, Apr 19, 2021 at 01:21:56PM +0100, Matthew Wilcox wrote:
> On Mon, Apr 19, 2021 at 02:56:17PM +0300, Mike Rapoport wrote:
> 
> > > With that fixed, you'll have a head page that you can use for testing,
> > > which means you don't need to test PageCompound() (because you know the
> > > page isn't PageTail), you can just test PageHead().
> > 
> > I can't say I follow you here. page_is_secretmem() is intended to be a
> > generic test, so I don't see how it will get PageHead() if it is called
> > from other places.
> 
> static inline bool head_is_secretmem(struct page *head)
> {
> 	if (PageHead(head))
> 		return false;
> 	...
> }
> 
> static inline bool page_is_secretmem(struct page *page)
> {
> 	if (PageTail(page))
> 		return false;
> 	return head_is_secretmem(page);
> }
> 
> (yes, calling it head is a misnomer, because it's not necessarily a head,
> it might be a base page, but until we have a name for pages which might
> be a head page or a base page, it'll have to do ...)

To me this looks less clean and readable and in the end we have an extra
check for PG_Head if that won't get optimized away.

Does not seem to me there would be a measurable difference, but if this is
important for future conversion to folio I don't mind using
{head,page}_is_secretmem.

-- 
Sincerely yours,
Mike.