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

Kirill A. Shutemov
2022-04-13T15:13:48Z

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.

> 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.

--
Kirill A. Shutemov