From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Subject: mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 1
_PAGE_PSE is used to distinguish between a truly non-present
(_PAGE_PRESENT=0) PMD, and a PMD which is undergoing a THP split and
should be treated as present.
But _PAGE_SWP_SOFT_DIRTY currently uses the _PAGE_PSE bit, which would
cause confusion between one of those PMDs undergoing a THP split, and a
soft-dirty PMD. Dropping _PAGE_PSE check in pmd_present() does not work
well, because it can hurt optimization of tlb handling in thp split.
Thus, we need to move the bit.
In the current kernel, bits 1-4 are not used in non-present format since
commit 00839ee3b299 ("x86/mm: Move swap offset/type up in PTE to work
around erratum"). So let's move _PAGE_SWP_SOFT_DIRTY to bit 1. Bit 7 is
used as reserved (always clear), so please don't use it for other purpose.
Link: http://lkml.kernel.org/r/20170717193955.20207-3-zi.yan@xxxxxxxx
Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Signed-off-by: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx>
Cc: David Nellans <dnellans@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/pgtable_64.h | 12 +++++++++---
arch/x86/include/asm/pgtable_types.h | 10 +++++-----
2 files changed, 14 insertions(+), 8 deletions(-)
diff -puN arch/x86/include/asm/pgtable_64.h~mm-x86-move-_page_swp_soft_dirty-from-bit-7-to-bit-1 arch/x86/include/asm/pgtable_64.h
--- a/arch/x86/include/asm/pgtable_64.h~mm-x86-move-_page_swp_soft_dirty-from-bit-7-to-bit-1
+++ a/arch/x86/include/asm/pgtable_64.h
@@ -180,15 +180,21 @@ static inline int pgd_large(pgd_t pgd) {
/*
* Encode and de-code a swap entry
*
- * | ... | 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 (9-13) |0|X|X|X| X| X|X|X|0| <- swp entry
+ * | ... | 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 (9-13) |0|0|X|X| X| X|X|SD|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
* there. We also need to avoid using A and D because of an
* erratum where they can be incorrectly set by hardware on
* non-present PTEs.
+ *
+ * SD (1) in swp entry is used to store soft dirty bit, which helps us
+ * remember soft dirty over page migration
+ *
+ * Bit 7 in swp entry should be 0 because pmd_present checks not only P,
+ * but also L and G.
*/
#define SWP_TYPE_FIRST_BIT (_PAGE_BIT_PROTNONE + 1)
#define SWP_TYPE_BITS 5
diff -puN arch/x86/include/asm/pgtable_types.h~mm-x86-move-_page_swp_soft_dirty-from-bit-7-to-bit-1 arch/x86/include/asm/pgtable_types.h
--- a/arch/x86/include/asm/pgtable_types.h~mm-x86-move-_page_swp_soft_dirty-from-bit-7-to-bit-1
+++ a/arch/x86/include/asm/pgtable_types.h
@@ -99,15 +99,15 @@
/*
* 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.
+ * with swap entry format. On x86 bits 1-4 are *not* involved
+ * into swap entry computation, but bit 7 is used for thp migration,
+ * so we borrow bit 1 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!
+ * mark if and only if the PTE/PMD has present bit clear!
*/
#ifdef CONFIG_MEM_SOFT_DIRTY
-#define _PAGE_SWP_SOFT_DIRTY _PAGE_PSE
+#define _PAGE_SWP_SOFT_DIRTY _PAGE_RW
#else
#define _PAGE_SWP_SOFT_DIRTY (_AT(pteval_t, 0))
#endif
_
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