x86: define _PAGE_NUMA by reusing software bits on the PMD and PTE levels

_PAGE_NUMA is currently an alias of _PROT_PROTNONE to trap NUMA hinting
faults on x86.  Care is taken such that _PAGE_NUMA is used only in
situations where the VMA flags distinguish between NUMA hinting faults
and prot_none faults.  This decision was x86-specific and conceptually
it is difficult requiring special casing to distinguish between PROTNONE
and NUMA ptes based on context.

Fundamentally, we only need the _PAGE_NUMA bit to tell the difference
between an entry that is really unmapped and a page that is protected
for NUMA hinting faults as if the PTE is not present then a fault will
be trapped.

Swap PTEs on x86-64 use the bits after _PAGE_GLOBAL for the offset.
This patch shrinks the maximum possible swap size and uses the bit to
uniquely distinguish between NUMA hinting ptes and swap ptes.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Anvin <hpa@zytor.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Steven Noonan <steven@uplinklabs.net>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 55 insertions, 36 deletions

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index b459ddf27d64..66276c1d23bb 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -131,7 +131,8 @@ static inline int pte_exec(pte_t pte)
 
 static inline int pte_special(pte_t pte)
 {
-        return pte_flags(pte) & _PAGE_SPECIAL;
+        return (pte_flags(pte) & (_PAGE_PRESENT|_PAGE_SPECIAL)) ==
+                                 (_PAGE_PRESENT|_PAGE_SPECIAL);
 }
 
 static inline unsigned long pte_pfn(pte_t pte)
@@ -452,6 +453,12 @@ static inline int pte_present(pte_t a)
                                _PAGE_NUMA);
 }
 
+#define pte_present_nonuma pte_present_nonuma
+static inline int pte_present_nonuma(pte_t a)
+{
+        return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE);
+}
+
 #define pte_accessible pte_accessible
 static inline bool pte_accessible(struct mm_struct *mm, pte_t a)
 {
@@ -860,19 +867,19 @@ static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
 
 static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
 {
-        VM_BUG_ON(pte_present(pte));
+        VM_BUG_ON(pte_present_nonuma(pte));
         return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
 }
 
 static inline int pte_swp_soft_dirty(pte_t pte)
 {
-        VM_BUG_ON(pte_present(pte));
+        VM_BUG_ON(pte_present_nonuma(pte));
         return pte_flags(pte) & _PAGE_SWP_SOFT_DIRTY;
 }
 
 static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
 {
-        VM_BUG_ON(pte_present(pte));
+        VM_BUG_ON(pte_present_nonuma(pte));
         return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);
 }
 
diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
index e22c1dbf7feb..6d6ecd09883c 100644
--- a/arch/x86/include/asm/pgtable_64.h
+++ b/arch/x86/include/asm/pgtable_64.h
@@ -145,8 +145,16 @@ static inline int pgd_large(pgd_t pgd) { return 0; }
 /* 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)
+#ifdef CONFIG_NUMA_BALANCING
+/* Automatic NUMA balancing needs to be distinguishable from swap entries */
+#define SWP_OFFSET_SHIFT (_PAGE_BIT_PROTNONE + 2)
+#else
 #define SWP_OFFSET_SHIFT (_PAGE_BIT_PROTNONE + 1)
+#endif
 #else
+#ifdef CONFIG_NUMA_BALANCING
+#error Incompatible format for automatic NUMA balancing
+#endif
 #define SWP_TYPE_BITS (_PAGE_BIT_PROTNONE - _PAGE_BIT_PRESENT - 1)
 #define SWP_OFFSET_SHIFT (_PAGE_BIT_FILE + 1)
 #endif
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index eb3d44945133..f216963760e5 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -16,15 +16,26 @@
 #define _PAGE_BIT_PSE           7       /* 4 MB (or 2MB) page */
 #define _PAGE_BIT_PAT           7       /* on 4KB pages */
 #define _PAGE_BIT_GLOBAL        8       /* Global TLB entry PPro+ */
-#define _PAGE_BIT_UNUSED1       9       /* available for programmer */
-#define _PAGE_BIT_IOMAP         10      /* flag used to indicate IO mapping */
-#define _PAGE_BIT_HIDDEN        11      /* hidden by kmemcheck */
+#define _PAGE_BIT_SOFTW1        9       /* available for programmer */
+#define _PAGE_BIT_SOFTW2        10      /* " */
+#define _PAGE_BIT_SOFTW3        11      /* " */
 #define _PAGE_BIT_PAT_LARGE     12      /* On 2MB or 1GB pages */
-#define _PAGE_BIT_SPECIAL       _PAGE_BIT_UNUSED1
-#define _PAGE_BIT_CPA_TEST      _PAGE_BIT_UNUSED1
-#define _PAGE_BIT_SPLITTING     _PAGE_BIT_UNUSED1 /* only valid on a PSE pmd */
+#define _PAGE_BIT_SPECIAL       _PAGE_BIT_SOFTW1
+#define _PAGE_BIT_CPA_TEST      _PAGE_BIT_SOFTW1
+#define _PAGE_BIT_SPLITTING     _PAGE_BIT_SOFTW2 /* only valid on a PSE pmd */
+#define _PAGE_BIT_IOMAP         _PAGE_BIT_SOFTW2 /* flag used to indicate IO mapping */
+#define _PAGE_BIT_HIDDEN        _PAGE_BIT_SOFTW3 /* hidden by kmemcheck */
+#define _PAGE_BIT_SOFT_DIRTY    _PAGE_BIT_SOFTW3 /* software dirty tracking */
 #define _PAGE_BIT_NX           63       /* No execute: only valid after cpuid check */
 
+/*
+ * Swap offsets on configurations that allow automatic NUMA balancing use the
+ * bits after _PAGE_BIT_GLOBAL. To uniquely distinguish NUMA hinting PTEs from
+ * swap entries, we use the first bit after _PAGE_BIT_GLOBAL and shrink the
+ * maximum possible swap space from 16TB to 8TB.
+ */
+#define _PAGE_BIT_NUMA          (_PAGE_BIT_GLOBAL+1)
+
 /* If _PAGE_BIT_PRESENT is clear, we use these: */
 /* - if the user mapped it with PROT_NONE; pte_present gives true */
 #define _PAGE_BIT_PROTNONE      _PAGE_BIT_GLOBAL
@@ -40,7 +51,7 @@
 #define _PAGE_DIRTY     (_AT(pteval_t, 1) << _PAGE_BIT_DIRTY)
 #define _PAGE_PSE       (_AT(pteval_t, 1) << _PAGE_BIT_PSE)
 #define _PAGE_GLOBAL    (_AT(pteval_t, 1) << _PAGE_BIT_GLOBAL)
-#define _PAGE_UNUSED1   (_AT(pteval_t, 1) << _PAGE_BIT_UNUSED1)
+#define _PAGE_SOFTW1    (_AT(pteval_t, 1) << _PAGE_BIT_SOFTW1)
 #define _PAGE_IOMAP     (_AT(pteval_t, 1) << _PAGE_BIT_IOMAP)
 #define _PAGE_PAT       (_AT(pteval_t, 1) << _PAGE_BIT_PAT)
 #define _PAGE_PAT_LARGE (_AT(pteval_t, 1) << _PAGE_BIT_PAT_LARGE)
@@ -61,8 +72,6 @@
  * they do not conflict with each other.
  */
 
-#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)
 #else
@@ -70,6 +79,21 @@
 #endif
 
 /*
+ * _PAGE_NUMA distinguishes between a numa hinting minor fault and a page
+ * that is not present. The hinting fault gathers numa placement statistics
+ * (see pte_numa()). The bit is always zero when the PTE is not present.
+ *
+ * The bit picked must be always zero when the pmd is present and not
+ * present, so that we don't lose information when we set it while
+ * atomically clearing the present bit.
+ */
+#ifdef CONFIG_NUMA_BALANCING
+#define _PAGE_NUMA      (_AT(pteval_t, 1) << _PAGE_BIT_NUMA)
+#else
+#define _PAGE_NUMA      (_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
@@ -94,26 +118,6 @@
 #define _PAGE_FILE      (_AT(pteval_t, 1) << _PAGE_BIT_FILE)
 #define _PAGE_PROTNONE  (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)
 
-/*
- * _PAGE_NUMA indicates that this page will trigger a numa hinting
- * minor page fault to gather numa placement statistics (see
- * pte_numa()). The bit picked (8) is within the range between
- * _PAGE_FILE (6) and _PAGE_PROTNONE (8) bits. Therefore, it doesn't
- * require changes to the swp entry format because that bit is always
- * zero when the pte is not present.
- *
- * The bit picked must be always zero when the pmd is present and not
- * present, so that we don't lose information when we set it while
- * atomically clearing the present bit.
- *
- * Because we shared the same bit (8) with _PAGE_PROTNONE this can be
- * interpreted as _PAGE_NUMA only in places that _PAGE_PROTNONE
- * couldn't reach, like handle_mm_fault() (see access_error in
- * arch/x86/mm/fault.c, the vma protection must not be PROT_NONE for
- * handle_mm_fault() to be invoked).
- */
-#define _PAGE_NUMA      _PAGE_PROTNONE
-
 #define _PAGE_TABLE     (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER |        \
                          _PAGE_ACCESSED | _PAGE_DIRTY)
 #define _KERNPG_TABLE   (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED |    \
@@ -122,8 +126,8 @@
 /* Set of bits not changed in pte_modify */
 #define _PAGE_CHG_MASK  (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |         \
                          _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY | \
-                         _PAGE_SOFT_DIRTY)
-#define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)
+                         _PAGE_SOFT_DIRTY | _PAGE_NUMA)
+#define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE | _PAGE_NUMA)
 
 #define _PAGE_CACHE_MASK        (_PAGE_PCD | _PAGE_PWT)
 #define _PAGE_CACHE_WB          (0)
diff --git a/arch/x86/mm/pageattr-test.c b/arch/x86/mm/pageattr-test.c
index 461bc8289024..6629f397b467 100644
--- a/arch/x86/mm/pageattr-test.c
+++ b/arch/x86/mm/pageattr-test.c
@@ -35,7 +35,7 @@ enum {
 
 static int pte_testbit(pte_t pte)
 {
-        return pte_flags(pte) & _PAGE_UNUSED1;
+        return pte_flags(pte) & _PAGE_SOFTW1;
 }
 
 struct split_state {