[PATCH] pagemap: update documentation

@@ -16,12 +16,17 @@ There are three components to pagemap:
     * Bits 0-4   swap type if swapped
     * Bits 5-54  swap offset if swapped
     * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
-    * Bit  56    page exlusively mapped
+    * Bit  56    page exclusively mapped (since 4.2)
     * Bits 57-60 zero
-    * Bit  61    page is file-page or shared-anon
+    * Bit  61    page is file-page or shared-anon (since 3.5)
     * Bit  62    page swapped
     * Bit  63    page present
 
+   Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs:
+   for unprivileged users from 4.0 till 4.2 open fails with -EPERM, starting
+   from from 4.2 PFN field is zeroed if user has no CAP_SYS_ADMIN capability.
+   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.
+
    If the page is not present but in swap, then the PFN contains an
    encoding of the swap file number and the page's offset into the
    swap. Unmapped pages return a null PFN. This allows determining

@@ -160,3 +165,8 @@ Other notes:
 Reading from any of the files will return -EINVAL if you are not starting
 the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
 into the file), or if the size of the read is not a multiple of 8 bytes.
+
+Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
+always 12 at most architectures). Since Linux 3.11 their meaning changes
+after first clear of soft-dirty bits. Since Linux 4.2 they are used for
+flags unconditionally.