linux-kernel - Re: [patch 2/2] [PATCH] mm: Save soft-dirty bits on file pages

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Date:	Wed, 14 Aug 2013 01:28:57 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, xemul@...allels.com, mpm@...enic.com,
	xiaoguangrong@...ux.vnet.ibm.com, mtosatti@...hat.com,
	kosaki.motohiro@...il.com, sfr@...b.auug.org.au,
	peterz@...radead.org, aneesh.kumar@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 2/2] [PATCH] mm: Save soft-dirty bits on file pages

On Tue, Aug 13, 2013 at 09:43:23AM -0700, H. Peter Anvin wrote:
> On 08/13/2013 08:37 AM, Cyrill Gorcunov wrote:
> >>
> >> Does it actually matter, generated-code-wise, or is the compiler smart
> >> enough to figure it out?  The reason I'm asking is because it makes the
> > 
> > gcc-4.7.2 is smart enough to suppress useless masking (ie ((1u << 31) - 1))
> > completely but I don't know if this can be assumed for all gcc series.
> > 
> 
> I would be highly surprised if it wasn't the case for any gcc we care about.

Does below one looks better? (Btw, what about the snippet we have there as well

#if _PAGE_BIT_FILE < _PAGE_BIT_PROTNONE
#define PTE_FILE_SHIFT2		(_PAGE_BIT_FILE + 1)
#define PTE_FILE_SHIFT3		(_PAGE_BIT_PROTNONE + 1)
#else
#define PTE_FILE_SHIFT2		(_PAGE_BIT_PROTNONE + 1)
#define PTE_FILE_SHIFT3		(_PAGE_BIT_FILE + 1)
#endif

where

#define _PAGE_BIT_PROTNONE	_PAGE_BIT_GLOBAL	-> 8
#define _PAGE_BIT_FILE		_PAGE_BIT_DIRTY		-> 6

so I wonder where the cases on x86 when _PAGE_BIT_FILE > _PAGE_BIT_PROTNONE,
what i'm missing here?)

---
 arch/x86/include/asm/pgtable-2level.h |   37 +++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 16 deletions(-)

Index: linux-2.6.git/arch/x86/include/asm/pgtable-2level.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/pgtable-2level.h
+++ linux-2.6.git/arch/x86/include/asm/pgtable-2level.h
@@ -55,8 +55,11 @@ static inline pmd_t native_pmdp_get_and_
 #define native_pmdp_get_and_clear(xp) native_local_pmdp_get_and_clear(xp)
 #endif
 
-#define _mfrob(v,r,m,l)		((((v) >> (r)) & (m)) << (l))
-#define __frob(v,r,l)		(((v) >> (r)) << (l))
+/*
+ * For readable bitfield manipulations.
+ */
+#define PTE_FILE_NOMASK		(-1U)
+#define __bfop(v,r,m,l)		((((v) >> (r)) & (m)) << (l))
 
 #ifdef CONFIG_MEM_SOFT_DIRTY
 
@@ -83,17 +86,17 @@ static inline pmd_t native_pmdp_get_and_
 #define PTE_FILE_LSHIFT4	(PTE_FILE_BITS1 + PTE_FILE_BITS2 + PTE_FILE_BITS3)
 
 #define pte_to_pgoff(pte)							    \
-	(_mfrob((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0)		  + \
-	 _mfrob((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
-	 _mfrob((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_MASK3, PTE_FILE_LSHIFT3) + \
-	 __frob((pte).pte_low, PTE_FILE_SHIFT4, PTE_FILE_LSHIFT4))
+	(__bfop((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0)		  + \
+	 __bfop((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
+	 __bfop((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_MASK3, PTE_FILE_LSHIFT3) + \
+	 __bfop((pte).pte_low, PTE_FILE_SHIFT4, PTE_FILE_NOMASK, PTE_FILE_LSHIFT4))
 
 #define pgoff_to_pte(off)							\
 	((pte_t) { .pte_low =							\
-	_mfrob(off,                0, PTE_FILE_MASK1, PTE_FILE_SHIFT1)	+	\
-	_mfrob(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2)	+	\
-	_mfrob(off, PTE_FILE_LSHIFT3, PTE_FILE_MASK3, PTE_FILE_SHIFT3)	+	\
-	__frob(off, PTE_FILE_LSHIFT4, PTE_FILE_SHIFT4)			+	\
+	__bfop(off,                0, PTE_FILE_MASK1, PTE_FILE_SHIFT1)	+	\
+	__bfop(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2)	+	\
+	__bfop(off, PTE_FILE_LSHIFT3, PTE_FILE_MASK3, PTE_FILE_SHIFT3)	+	\
+	__bfop(off, PTE_FILE_LSHIFT4, PTE_FILE_NOMASK, PTE_FILE_SHIFT4)	+	\
 	_PAGE_FILE })
 
 #else /* CONFIG_MEM_SOFT_DIRTY */
@@ -121,19 +124,21 @@ static inline pmd_t native_pmdp_get_and_
 #define PTE_FILE_LSHIFT3	(PTE_FILE_BITS1 + PTE_FILE_BITS2)
 
 #define pte_to_pgoff(pte)							    \
-	(_mfrob((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0)		  + \
-	 _mfrob((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
-	 __frob((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_LSHIFT3))
+	(__bfop((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0)		  + \
+	 __bfop((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
+	 __bfop((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_NOMASK, PTE_FILE_LSHIFT3))
 
 #define pgoff_to_pte(off)							\
 	((pte_t) { .pte_low =							\
-	_mfrob(off,                0, PTE_FILE_MASK1, PTE_FILE_SHIFT1)	+	\
-	_mfrob(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2)	+	\
-	__frob(off, PTE_FILE_LSHIFT3, PTE_FILE_SHIFT3)			+	\
+	__bfop(off,                0, PTE_FILE_MASK1, PTE_FILE_SHIFT1)	+	\
+	__bfop(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2)	+	\
+	__bfop(off, PTE_FILE_LSHIFT3, PTE_FILE_NOMASK, PTE_FILE_SHIFT3)	+	\
 	_PAGE_FILE })
 
 #endif /* CONFIG_MEM_SOFT_DIRTY */
 
+#undef __bfop
+
 /* Encode and de-code a swap entry */
 #if _PAGE_BIT_FILE < _PAGE_BIT_PROTNONE
 #define SWP_TYPE_BITS (_PAGE_BIT_FILE - _PAGE_BIT_PRESENT - 1)
--
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