[PATCH -mm] docs: Document soft dirty behaviour for freshly createdmemory regions

[Cyrill Gorcunov]

Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matt Mackall <mpm@xxxxxxxxxxx>
Cc: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx>
Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
---
 Documentation/vm/soft-dirty.txt |    7 +++++++
 1 file changed, 7 insertions(+)

Index: linux-2.6.git/Documentation/vm/soft-dirty.txt
===================================================================
--- linux-2.6.git.orig/Documentation/vm/soft-dirty.txt
+++ linux-2.6.git/Documentation/vm/soft-dirty.txt
@@ -28,6 +28,13 @@ This is so, since the pages are still ma
 the kernel does is finds this fact out and puts both writable and soft-dirty
 bits on the PTE.
 
+  While in most cases tracking memory changes by #PF-s is more than enough
+there is still a scenario when we can loose soft dirty bit -- a task does
+unmap previously mapped memory region and then maps new one exactly at the
+same place. When unmap called the kernel internally clears PTEs values
+including soft dirty bit. To notify user space application about such
+memory region renewal the kernel always mark new memory regions (and
+expanded regions) as soft dirtified.
 
   This feature is actively used by the checkpoint-restore project. You
 can find more details about it on http://criu.org
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