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[PATCH] x86, mm: Fix swap entry comment and macro

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  [PATCH] x86, mm: Fix swap entry comment and macro Dave Hansen <dave@sr71.net> - 2016-08-10 20:20 +0200

#1459390 — [PATCH] x86, mm: Fix swap entry comment and macro

From: Dave Hansen <dave.hansen@linux.intel.com>

A recent patch changed the format of a swap PTE.

The comment explaining the format of the swap PTE is wrong about
the bits used for the swap type field.  Amusingly, the ASCII art
and the patch description are correct, but the comment itself
is wrong.

As I was looking at this, I also noticed that the
SWP_OFFSET_FIRST_BIT has an off-by-one error.  This does not
really hurt anything.  It just wasted a bit of space in the PTE,
giving us 2^59 bytes of addressable space in our swapfiles
instead of 2^60.  But, it doesn't match with the comments, and it
wastes a bit of space, so fix it.

Fixes: 00839ee3b299 ("x86/mm: Move swap offset/type up in PTE to work around erratum")
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---

 b/arch/x86/include/asm/pgtable_64.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff -puN arch/x86/include/asm/pgtable_64.h~x86-pgtables-fix-comment arch/x86/include/asm/pgtable_64.h
--- a/arch/x86/include/asm/pgtable_64.h~x86-pgtables-fix-comment	2016-08-09 16:22:46.631898618 -0700
+++ b/arch/x86/include/asm/pgtable_64.h	2016-08-09 16:26:29.744037506 -0700
@@ -145,7 +145,7 @@ static inline int pgd_large(pgd_t pgd) {
  *
  * |     ...            | 11| 10|  9|8|7|6|5| 4| 3|2|1|0| <- bit number
  * |     ...            |SW3|SW2|SW1|G|L|D|A|CD|WT|U|W|P| <- bit names
- * | OFFSET (14->63) | TYPE (10-13) |0|X|X|X| X| X|X|X|0| <- swp entry
+ * | OFFSET (14->63) | TYPE (9-13)  |0|X|X|X| X| X|X|X|0| <- swp entry
  *
  * G (8) is aliased and used as a PROT_NONE indicator for
  * !present ptes.  We need to start storing swap entries above
@@ -156,7 +156,7 @@ static inline int pgd_large(pgd_t pgd) {
 #define SWP_TYPE_FIRST_BIT (_PAGE_BIT_PROTNONE + 1)
 #define SWP_TYPE_BITS 5
 /* Place the offset above the type: */
-#define SWP_OFFSET_FIRST_BIT (SWP_TYPE_FIRST_BIT + SWP_TYPE_BITS + 1)
+#define SWP_OFFSET_FIRST_BIT (SWP_TYPE_FIRST_BIT + SWP_TYPE_BITS)
 
 #define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > SWP_TYPE_BITS)
 
_

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