proc.5: Add info about /proc/[pid]/map_files directory (v2)

It was added into Linux v3.3 and provides info about files being mmap-ed in a way _very_ similar to how /proc/[pid]/fd works. Thus I thought that the best place for the map_files' doc is where where the fd's doc is.

v2: Added examples of how links look like and dependency on kernel config option CONFIG_CHECKPOINT_RESTORE.

Signed-off-by Pavel Emelyanov <[email protected]>

---

man5/proc.5 +29 -0

index 520c5fc..1bfc63c 100644
@@ -286,6 +286,42 @@ Thus the example command above could be written as:
 .RB "$" " foobar \-i /dev/stdin \-o /dev/stdout ..."
 .fi
 .in
+.TP
+.IR /proc/[pid]/map_files/ " (since kernel 3.3)
+This subdirectory contains entries corresponding to memory mapped
+files (see
+.BR mmap (2)).
+Entries are named by memory region start and end pair,
+and are symbolic links to the files themselves. Like this
+.in +4n
+.nf
+
+.RB "$" " ls -l /proc/self/map_files/"
+lr--------. 1 root root 64 Apr 16 21:31 3252e00000-3252e20000 -> /usr/lib64/ld-2.15.so
+\&...
+.fi
+.in
+
+Although these entries are present for memory regions that were
+mapped with
+.BR MAP_FILE
+flag, the way anonymous shared memory (regions created with
+.B MAP_ANON | MAP_SHARED
+flags)
+is implemented in Linux
+makes such regions also appear on this directory. In that case
+the target file would be the deleted /dev/zero one:
+.in +4n
+.nf
+
+.RB