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

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

[Dave Hansen]

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)
 
_

[tip:x86/urgent] x86/mm: Fix swap entry comment and macro

[tip-bot for Dave Hansen]

Commit-ID:  ace7fab7a6cdd363a615ec537f2aa94dbc761ee2
Gitweb:     http://git.kernel.org/tip/ace7fab7a6cdd363a615ec537f2aa94dbc761ee2
Author:     Dave Hansen <dave.hansen@linux.intel.com>
AuthorDate: Wed, 10 Aug 2016 10:23:25 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 11 Aug 2016 11:04:10 +0200

x86/mm: Fix swap entry comment and macro

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.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Toshi Kani <toshi.kani@hp.com>
Fixes: 00839ee3b299 ("x86/mm: Move swap offset/type up in PTE to work around erratum")
Link: http://lkml.kernel.org/r/20160810172325.E56AD7DA@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/pgtable_64.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
index 7e8ec7a..1cc82ec 100644
--- a/arch/x86/include/asm/pgtable_64.h
+++ b/arch/x86/include/asm/pgtable_64.h
@@ -145,7 +145,7 @@ static inline int pgd_large(pgd_t pgd) { return 0; }
  *
  * |     ...            | 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) { return 0; }
 #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)