smaps: add clear_refs file to clear reference

Adds /proc/pid/clear_refs.  When any non-zero number is written to this file,
pte_mkold() and ClearPageReferenced() is called for each pte and its
corresponding page, respectively, in that task's VMAs.  This file is only
writable by the user who owns the task.

It is now possible to measure _approximately_ how much memory a task is using
by clearing the reference bits with

	echo 1 > /proc/pid/clear_refs

and checking the reference count for each VMA from the /proc/pid/smaps output
at a measured time interval.  For example, to observe the approximate change
in memory footprint for a task, write a script that clears the references
(echo 1 > /proc/pid/clear_refs), sleeps, and then greps for Pgs_Referenced and
extracts the size in kB.  Add the sizes for each VMA together for the total
referenced footprint.  Moments later, repeat the process and observe the
difference.

For example, using an efficient Mozilla:

	accumulated time		referenced memory
	----------------		-----------------
		 0 s				 408 kB
		 1 s				 408 kB
		 2 s				 556 kB
		 3 s				1028 kB
		 4 s				 872 kB
		 5 s				1956 kB
		 6 s				 416 kB
		 7 s				1560 kB
		 8 s				2336 kB
		 9 s				1044 kB
		10 s				 416 kB

This is a valuable tool to get an approximate measurement of the memory
footprint for a task.

Cc: Hugh Dickins <hugh@veritas.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Christoph Lameter <clameter@sgi.com>
Signed-off-by: David Rientjes <rientjes@google.com>
[akpm@linux-foundation.org: build fixes]
[mpm@selenic.com: rename for_each_pmd]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 15 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 7aaf09b86a55..3f4b226572e7 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -122,21 +122,22 @@ subdirectory has the entries listed in Table 1-1.
 
 Table 1-1: Process specific entries in /proc 
 ..............................................................................
- File    Content                                        
- cmdline Command line arguments                         
- cpu	 Current and last cpu in which it was executed		(2.4)(smp)
- cwd	 Link to the current working directory
- environ Values of environment variables      
- exe	 Link to the executable of this process
- fd      Directory, which contains all file descriptors 
- maps	 Memory maps to executables and library files		(2.4)
- mem     Memory held by this process                    
- root	 Link to the root directory of this process
- stat    Process status                                 
- statm   Process memory status information              
- status  Process status in human readable form          
- wchan   If CONFIG_KALLSYMS is set, a pre-decoded wchan
- smaps	 Extension based on maps, presenting the rss size for each mapped file
+ File		Content
+ clear_refs	Clears page referenced bits shown in smaps output
+ cmdline	Command line arguments
+ cpu		Current and last cpu in which it was executed	(2.4)(smp)
+ cwd		Link to the current working directory
+ environ	Values of environment variables
+ exe		Link to the executable of this process
+ fd		Directory, which contains all file descriptors
+ maps		Memory maps to executables and library files	(2.4)
+ mem		Memory held by this process
+ root		Link to the root directory of this process
+ stat		Process status
+ statm		Process memory status information
+ status		Process status in human readable form
+ wchan		If CONFIG_KALLSYMS is set, a pre-decoded wchan
+ smaps		Extension based on maps, the rss size for each mapped file
 ..............................................................................
 
 For example, to get the status information of a process, all you have to do is