[rfc 3/3] mm: pgtable -- Use _PAGE_SOFT_DIRTY for swap entries

[Cyrill Gorcunov]

Since we support soft-dirty on x86-64 now we can release _PAGE_PSE
bit used to track dirty swap entries and reuse ealready existing
_PAGE_SOFT_DIRTY.

Thus for all soft-dirty needs we use same pte bit.

CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Mel Gorman <mgorman@xxxxxxx>
CC: Peter Anvin <hpa@xxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Steven Noonan <steven@xxxxxxxxxxxxxx>
CC: Rik van Riel <riel@xxxxxxxxxx>
CC: David Vrabel <david.vrabel@xxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
---
 arch/x86/include/asm/pgtable_64.h    |   12 ++++++++++--
 arch/x86/include/asm/pgtable_types.h |   19 ++++---------------
 2 files changed, 14 insertions(+), 17 deletions(-)

Index: linux-2.6.git/arch/x86/include/asm/pgtable_64.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/pgtable_64.h
+++ linux-2.6.git/arch/x86/include/asm/pgtable_64.h
@@ -142,9 +142,17 @@ static inline int pgd_large(pgd_t pgd) {
 #define pte_offset_map(dir, address) pte_offset_kernel((dir), (address))
 #define pte_unmap(pte) ((void)(pte))/* NOP */
 
-/* Encode and de-code a swap entry */
+/*
+ * Encode and de-code a swap entry. When soft-dirty memory tracker is
+ * enabled we need to borrow _PAGE_BIT_SOFT_DIRTY bit for own needs,
+ * which limits the max size of swap partiotion about to 1T.
+ */
 #define SWP_TYPE_BITS (_PAGE_BIT_FILE - _PAGE_BIT_PRESENT - 1)
-#define SWP_OFFSET_SHIFT (_PAGE_BIT_PROTNONE + 1)
+#ifdef CONFIG_MEM_SOFT_DIRTY
+# define SWP_OFFSET_SHIFT (_PAGE_BIT_SOFT_DIRTY + 1)
+#else
+# define SWP_OFFSET_SHIFT (_PAGE_BIT_PROTNONE + 1)
+#endif
 
 #define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > SWP_TYPE_BITS)
 
Index: linux-2.6.git/arch/x86/include/asm/pgtable_types.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/pgtable_types.h
+++ linux-2.6.git/arch/x86/include/asm/pgtable_types.h
@@ -59,29 +59,18 @@
  * The same hidden bit is used by kmemcheck, but since kmemcheck
  * works on kernel pages while soft-dirty engine on user space,
  * they do not conflict with each other.
+ *
+ * Because soft-dirty is limited to x86-64 only we can reuse this
+ * bit to track swap entries as well.
  */
 
 #define _PAGE_BIT_SOFT_DIRTY	_PAGE_BIT_HIDDEN
 
 #ifdef CONFIG_MEM_SOFT_DIRTY
 #define _PAGE_SOFT_DIRTY	(_AT(pteval_t, 1) << _PAGE_BIT_SOFT_DIRTY)
+#define _PAGE_SWP_SOFT_DIRTY	_PAGE_SOFT_DIRTY
 #else
 #define _PAGE_SOFT_DIRTY	(_AT(pteval_t, 0))
-#endif
-
-/*
- * Tracking soft dirty bit when a page goes to a swap is tricky.
- * We need a bit which can be stored in pte _and_ not conflict
- * with swap entry format. On x86 bits 6 and 7 are *not* involved
- * into swap entry computation, but bit 6 is used for nonlinear
- * file mapping, so we borrow bit 7 for soft dirty tracking.
- *
- * Please note that this bit must be treated as swap dirty page
- * mark if and only if the PTE has present bit clear!
- */
-#ifdef CONFIG_MEM_SOFT_DIRTY
-#define _PAGE_SWP_SOFT_DIRTY	_PAGE_PSE
-#else
 #define _PAGE_SWP_SOFT_DIRTY	(_AT(pteval_t, 0))
 #endif
 

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