linux-kernel - Re: [PATCH RFC 2/2] mm: introdule compound_head_by

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Message-ID: <CAHz2CGWJjU5U=pDuyzX=L+gev4cpNxkjCnVAvRpY=vO35tBDLg@mail.gmail.com>
Date:	Tue, 29 Apr 2014 00:23:41 +0800
From:	Jianyu Zhan <nasa4836@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	kirill.shutemov@...ux.intel.com, Rik van Riel <riel@...hat.com>,
	Jiang Liu <liuj97@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>, sasha.levin@...cle.com,
	liwanp@...ux.vnet.ibm.com, khalid.aziz@...cle.com,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 2/2] mm: introdule compound_head_by_tail()

On Mon, Apr 28, 2014 at 11:55 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> What version,

the code snipt in question is extracted from v3.15-rc3.


for the (1) check in previous email, its assembled code looks like:

       --- (1) snipt ---
       mov    (%rdi),%rax                               (a)
       test   $0x80,%ah                                  (b)
       jne    754 <put_compound_page+0x74> (c)
       --- (1) snipt ---

      (a) %rdi is the struct page pointer
      (b) check if PG_tail(0x80) set(likely not set, we tell the compiler)
      (c) if set, jump; not set, fall through (good, credit to  our hint)

===================================================

for the (3) check in previous email, its assembled code looks like:

        --- (3) snipt ---
        mov    (%rdi),%rax                                    (A)
        mov    %rdi,%r12
        test   $0x80,%ah                                      (B)
        jne    8f8 <put_compound_page+0x218>     (C)
        --- (3) snipt ---

      (A) %rdi is the struct page pointer
      (B) check if PG_tail(0x80) set(likely set in this case, but we
tell compiler unlikely)
      (C) if set, jump; not set, fall through (god! it would better
not jump if set,  but we
           tell compiler unlikely, so it happily did as we told it)


# all code are compiled by gcc (GCC) 4.8.2

> and why didn't your changelog include this useful information?

Sorry, I would have done so.  I will resend the patch.

Thanks,
Jianyu Zhan
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