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

[David Rientjes <rientjes@google.com>]

On Wed, 11 Jun 2014, Vlastimil Babka wrote:

> > I hate to belabor this point, but I think gcc does treat it differently.
> > If you look at the assembly comparing your patch to if you do
> > 
> > 	unsigned long freepage_order = ACCESS_ONCE(page_private(page));
> > 
> > instead, then if you enable annotation you'll see that gcc treats the
> > store as page_x->D.y.private in your patch vs. MEM[(volatile long unsigned
> > int *)page_x + 48B] with the above.
> 
> Hm sure you compiled a version that used page_order_unsafe() and not
> page_order()? Because I do see:
> 
> MEM[(volatile long unsigned int *)valid_page_114 + 48B];
> 
> That's gcc 4.8.1, but our gcc guy said he tried 4.5+ and all was like this.
> And that it would be a gcc bug if not.
> He also did a test where page_order was called twice in one function and
> page_order_unsafe twice in another function. page_order() was reduced to a
> single access in the assembly, page_order_unsafe were two accesses.
> 

Ok, and I won't continue to push the point.  I think the lockless 
suitable_migration_target() call that looks at page_order() is fine in the 
free scanner since we use it as a racy check, but it might benefit from 
either a comment describing the behavior or a sanity check for 
page_order(page) <= MAX_ORDER as you've done before.