Re: [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page_map

Kirill A. Shutemov
2014-06-16T21:00:29Z

On Mon, Jun 16, 2014 at 11:49:34PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jun 16, 2014 at 03:35:48PM -0400, Waiman Long wrote:
> > In the __split_huge_page_map() function, the check for
> > page_mapcount(page) is invariant within the for loop. Because of the
> > fact that the macro is implemented using atomic_read(), the redundant
> > check cannot be optimized away by the compiler leading to unnecessary
> > read to the page structure.

And atomic_read() is *not* atomic operation. It's implemented as
dereferencing though cast to volatile, which suppress compiler
optimization, but doesn't affect what CPU can do with the variable.

So I doubt difference will be measurable anywhere.