UBUNTU: SAUCE: Sync pgtable_64.h with upstream stable

BugLink: https://bugs.launchpad.net/bugs/1792174

The delta was only in comments or blank lines. So this brings the
file (arch/x86/include/asm/pgtable_64.h) into sync with 4.4.148.
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/include/asm/pgtable_64.h

@@ -163,12 +163,33 @@ static inline int pgd_large(pgd_t pgd) { return 0; }
 #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
+ *
+ * |     ...            | 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
+ * | TYPE (59-63) | ~OFFSET (9-58)  |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.
+ *
+ * The offset is inverted by a binary not operation to make the high
+ * physical bits set.
+ */
 #define SWP_TYPE_BITS		5
+
 #define SWP_OFFSET_FIRST_BIT	(_PAGE_BIT_PROTNONE + 1)
-/* We always extract/encode the offset by shifting it all the way up,
- * and then down again
- */
+
+/* We always extract/encode the offset by shifting it all the way up, and then down again */
 #define SWP_OFFSET_SHIFT	(SWP_OFFSET_FIRST_BIT+SWP_TYPE_BITS)
 
 #define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > SWP_TYPE_BITS)
@@ -180,7 +201,7 @@ static inline int pgd_large(pgd_t pgd) { return 0; }
 #define __swp_offset(x) (~(x).val << SWP_TYPE_BITS >> SWP_OFFSET_SHIFT)
 
 /*
- * Shift the offset up "too far" by TYPE bits, then down again.
+ * Shift the offset up "too far" by TYPE bits, then down again
  * The offset is inverted by a binary not operation to make the high
  * physical bits set.
  */
@@ -217,4 +238,5 @@ extern void init_extra_mapping_wb(unsigned long phys, unsigned long size);
 #include <asm/pgtable-invert.h>
 
 #endif /* !__ASSEMBLY__ */
+
 #endif /* _ASM_X86_PGTABLE_64_H */