linux-kernel - [PATCHv4 7/7] mm: use 'unsigned int' for compound_dtor/compound

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Message-Id: <1440683961-32839-8-git-send-email-kirill.shutemov@linux.intel.com>
Date:	Thu, 27 Aug 2015 16:59:21 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv4 7/7] mm: use 'unsigned int' for compound_dtor/compound_order on 64BIT

On 64 bit system we have enough space in struct page to encode
compound_dtor and compound_order with unsigned int.

On x86-64 it leads to slightly smaller code size due usesage of plain
MOV instead of MOVZX (zero-extended move) or similar effect.

allyesconfig:

   text	   data	    bss	    dec	    hex	filename
159520446	48146736	72196096	279863278	10ae5fee	vmlinux.pre
159520382	48146736	72196096	279863214	10ae5fae	vmlinux.post

On other architectures without native support of 16-bit data types the
difference can be bigger.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 include/linux/mm_types.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index ecaf3b1d0216..39b0db74ba5e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -150,8 +150,13 @@ struct page {
 		/* First tail page of compound page */
 		struct {
 			unsigned long compound_head; /* If bit zero is set */
+#ifdef CONFIG_64BIT
+			unsigned int compound_dtor;
+			unsigned int compound_order;
+#else
 			unsigned short int compound_dtor;
 			unsigned short int compound_order;
+#endif
 		};
 
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
-- 
2.5.0

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