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

[David Rientjes]

On Tue, 10 Jun 2014, Vlastimil Babka wrote:

> > I think the compiler is allowed to turn this into
> > 
> > 	if (ACCESS_ONCE(page_private(page)) > 0 &&
> > 	    ACCESS_ONCE(page_private(page)) < MAX_ORDER)
> > 		low_pfn += (1UL << ACCESS_ONCE(page_private(page))) - 1;
> > 
> > since the inline function has a return value of unsigned long but gcc may
> > not do this.  I think
> > 
> > 	/*
> > 	 * Big fat comment describing why we're using ACCESS_ONCE(), that
> > 	 * we're ok to race, and that this is meaningful only because of
> > 	 * the previous PageBuddy() check.
> > 	 */
> > 	unsigned long pageblock_order = ACCESS_ONCE(page_private(page));
> > 
> > is better.
> 
> I've talked about it with a gcc guy and (although he didn't actually see the
> code so it might be due to me not explaining it perfectly), the compiler will
> inline page_order_unsafe() so that there's effectively.
> 
> unsigned long freepage_order = ACCESS_ONCE(page_private(page));
> 
> and now it cannot just replace all freepage_order occurences with new
> page_private() accesses. So thanks to the inlining, the volatile qualification
> propagates to where it matters. It makes sense to me, but if it's according to
> standard or gcc specific, I don't know.
> 

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.

I don't have the ability to prove that all versions of gcc optimization 
will not choose to reaccess page_private(page) here, but it does show that 
at least gcc 4.6.3 does not consider them to be equivalents.
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