Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

[Mike Rapoport]

On Wed, Apr 13, 2022 at 06:15:17PM +0300, Kirill A. Shutemov wrote:
> On Wed, Apr 13, 2022 at 05:48:09PM +0300, Mike Rapoport wrote:
> > On Wed, Apr 13, 2022 at 02:40:01PM +0300, Kirill A. Shutemov wrote:
> > > On Mon, Apr 11, 2022 at 01:07:29PM +0300, Mike Rapoport wrote:
> > > > On Sun, Apr 10, 2022 at 11:38:08PM -0700, Dave Hansen wrote:
> > > > > On 4/9/22 08:54, Kirill A. Shutemov wrote:
> > > > > > On Fri, Apr 08, 2022 at 11:55:43AM -0700, Dave Hansen wrote:
> > > > > 
> > > > > >>>  	if (fpi_flags & FPI_TO_TAIL)
> > > > > >>>  		to_tail = true;
> > > > > >>>  	else if (is_shuffle_order(order))
> > > > > >>> @@ -1149,7 +1192,8 @@ static inline void __free_one_page(struct page *page,
> > > > > >>>  static inline bool page_expected_state(struct page *page,
> > > > > >>>  					unsigned long check_flags)
> > > > > >>>  {
> > > > > >>> -	if (unlikely(atomic_read(&page->_mapcount) != -1))
> > > > > >>> +	if (unlikely(atomic_read(&page->_mapcount) != -1) &&
> > > > > >>> +	    !PageUnaccepted(page))
> > > > > >>>  		return false;
> > > > > >>
> > > > > >> That probably deserves a comment, and maybe its own if() statement.
> > > > > > 
> > > > > > Own if does not work. PageUnaccepted() is encoded in _mapcount.
> > > > > > 
> > > > > > What about this:
> > > > > > 
> > > > > > 	/*
> > > > > > 	 * page->_mapcount is expected to be -1.
> > > > > > 	 *
> > > > > > 	 * There is an exception for PageUnaccepted(). The page type can be set
> > > > > > 	 * for pages on free list. Page types are encoded in _mapcount.
> > > > > > 	 *
> > > > > > 	 * PageUnaccepted() will get cleared in post_alloc_hook().
> > > > > > 	 */
> > > > > > 	if (unlikely((atomic_read(&page->_mapcount) | PG_unaccepted) != -1))
> > > > 
> > > > Maybe I'm missing something, but isn't this true for any PageType?
> > > 
> > > PG_buddy gets clear on remove from the free list, before the chec.
> > > 
> > > PG_offline and PG_table pages are never on free lists.
> > 
> > Right, this will work 'cause PageType is inverted. I still think this
> > condition is hard to parse and I liked the old variant with
> > !PageUnaccepted() better.
> 
> Well the old way to deal with PageUnaccepted() had a flaw: if the page is
> PageUnaccepted() it will allow any other page types to pass here. Like
> PG_unaccepted + PG_buddy will slide here.

It seems to me that there was an implicit assumption that page types are
exclusive and PG_unaccepted would break it.
 
> > Maybe if we wrap the whole construct in a helper it will be less eye
> > hurting.
> 
> Hm. Any suggestion how such helper could look like? Cannot think of
> anything sane.

Me neither :(

How about updating the comment to be

	/*
	 * The page must not be mapped to userspace and must not have a
	 * PageType other than PageUnaccepted.
	 * This means that page->_mapcount must be -1 or have only
	 * PG_unaccepted bit cleared.
	 */
	if (unlikely((atomic_read(&page->_mapcount) | PG_unaccepted) != -1))
 
> -- 
>  Kirill A. Shutemov

-- 
Sincerely yours,
Mike.