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

David Rientjes
2014-06-12T00:21:42Z

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.