Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner

[David Rientjes]

On Fri, 6 Jun 2014, Vlastimil Babka wrote:

> >> > > diff --git a/mm/internal.h b/mm/internal.h
> >> > > index 1a8a0d4..6aa1f74 100644
> >> > > --- a/mm/internal.h
> >> > > +++ b/mm/internal.h
> >> > > @@ -164,7 +164,8 @@ isolate_migratepages_range(struct zone *zone, struct
> >> > > compact_control *cc,
> >> > >    * general, page_zone(page)->lock must be held by the caller to prevent
> >> > > the
> >> > >    * page from being allocated in parallel and returning garbage as the
> >> > > order.
> >> > >    * If a caller does not hold page_zone(page)->lock, it must guarantee
> >> > > that the
> >> > > - * page cannot be allocated or merged in parallel.
> >> > > + * page cannot be allocated or merged in parallel. Alternatively, it must
> >> > > + * handle invalid values gracefully, and use page_order_unsafe() below.
> >> > >    */
> >> > >   static inline unsigned long page_order(struct page *page)
> >> > >   {
> >> > > @@ -172,6 +173,23 @@ static inline unsigned long page_order(struct page
> >> > > *page)
> >> > >   	return page_private(page);
> >> > >   }
> >> > > 
> >> > > +/*
> >> > > + * Like page_order(), but for callers who cannot afford to hold the zone
> >> > > lock,
> >> > > + * and handle invalid values gracefully. ACCESS_ONCE is used so that if
> >> > > the
> >> > > + * caller assigns the result into a local variable and e.g. tests it for
> >> > > valid
> >> > > + * range  before using, the compiler cannot decide to remove the variable
> >> > > and
> >> > > + * inline the function multiple times, potentially observing different
> >> > > values
> >> > > + * in the tests and the actual use of the result.
> >> > > + */
> >> > > +static inline unsigned long page_order_unsafe(struct page *page)
> >> > > +{
> >> > > +	/*
> >> > > +	 * PageBuddy() should be checked by the caller to minimize race
> >> > > window,
> >> > > +	 * and invalid values must be handled gracefully.
> >> > > +	 */
> >> > > +	return ACCESS_ONCE(page_private(page));
> >> > > +}
> >> > > +
> >> > >   /* mm/util.c */
> >> > >   void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
> >> > >   		struct vm_area_struct *prev, struct rb_node *rb_parent);
> >> > 
> >> > I don't like this change at all, I don't think we should have header
> >> > functions that imply the context in which the function will be called.  I
> >> > think it would make much more sense to just do
> >> > ACCESS_ONCE(page_order(page)) in the migration scanner with a comment.
> >> 
> >> But that won't compile. It would have to be converted to a #define, unless
> >> there's some trick I don't know. Sure I would hope this could be done cleaner
> >> somehow.
> >> 
> > 
> > Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner 
> 
> Hm but that's breaking the abstraction of page_order(). I don't know if it's
> worse to create a new variant of page_order() or to do this. BTW, seems like
> next_active_pageblock() in memory-hotplug.c should use this variant too.
> 

The compiler seems free to disregard the access of a volatile object above 
because the return value of the inline function is unsigned long.  What's 
the difference between unsigned long order = page_order_unsafe(page) and
unsigned long order = (unsigned long)ACCESS_ONCE(page_private(page)) and 
the compiler being able to reaccess page_private() because the result is 
no longer volatile qualified?
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