Re: [PATCH RFC 2/2] mm: introdule compound_head_by_tail()

[Jianyu Zhan <nasa4836@gmail.com>]

Hi, Michal,

On Mon, Apr 28, 2014 at 10:54 PM, Michal Hocko <mhocko@suse.cz> wrote:
> I really fail to see how that helps. compound_head is inlined and the
> compiler should be clever enough to optimize the code properly. I
> haven't tried that to be honest but this looks like it only adds a code
> without any good reason. And I really hate the new name as well. What
> does it suppose to mean?

the code in question is as below:

--- snipt ----
if (likely(!PageTail(page))) {                  <------  (1)
                if (put_page_testzero(page)) {
                        /*
                        ¦* By the time all refcounts have been released
                        ¦* split_huge_page cannot run anymore from under us.
                        ¦*/
                        if (PageHead(page))
                                __put_compound_page(page);
                        else
                                __put_single_page(page);
                }
                return;
}

/* __split_huge_page_refcount can run under us */
page_head = compound_head(page);        <------------ (2)
--- snipt ---

if at (1) ,  we fail the check, this means page is *likely* a tail page.

Then at (2), yes, compoud_head(page) is inlined, it is :

--- snipt ---
static inline struct page *compound_head(struct page *page)
{
          if (unlikely(PageTail(page))) {           <----------- (3)
              struct page *head = page->first_page;

                smp_rmb();
                if (likely(PageTail(page)))
                        return head;
        }
        return page;
}
--- snipt ---

here, the (3) unlikely in the case is  a negative hint, because it
is *likely* a tail page. So the check (3) in this case is not good,
so I introduce a helper for this case.

Actually, I checked the assembled code, the compiler is _not_
so smart to recognize this case. It just does optimization as
the hint unlikely() told it.



Thanks,
Jianyu Zhan

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