/proc/pid/smaps: export amount of anonymous memory in a mapping

Export the number of anonymous pages in a mapping via smaps.

Even the private pages in a mapping backed by a file, would be marked as
anonymous, when they are modified. Export this information to user-space via
smaps.

Exporting this count will help gdb to make a better decision on which
areas need to be dumped in its coredump; and should be useful to others
studying the memory usage of a process.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -370,6 +370,7 @@ Shared_Dirty:          0 kB
 Private_Clean:         0 kB
 Private_Dirty:         0 kB
 Referenced:          892 kB
+Anonymous:             0 kB
 Swap:                  0 kB
 KernelPageSize:        4 kB
 MMUPageSize:           4 kB
@@ -378,9 +379,15 @@ The first of these lines shows the same information as is displayed for the
 mapping in /proc/PID/maps.  The remaining lines show the size of the mapping
 (size), the amount of the mapping that is currently resident in RAM (RSS), the
 process' proportional share of this mapping (PSS), the number of clean and
-dirty shared pages in the mapping, and the number of clean and dirty private
-pages in the mapping.  The "Referenced" indicates the amount of memory
-currently marked as referenced or accessed.
+dirty private pages in the mapping.  Note that even a page which is part of a
+MAP_SHARED mapping, but has only a single pte mapped, i.e.  is currently used
+by only one process, is accounted as private and not as shared.  "Referenced"
+indicates the amount of memory currently marked as referenced or accessed.
+"Anonymous" shows the amount of memory that does not belong to any file.  Even
+a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
+and a page is modified, the file page is replaced by a private anonymous copy.
+"Swap" shows how much would-be-anonymous memory is also used, but out on
+swap.
 
 This file is only present if the CONFIG_MMU kernel configuration option is
 enabled.