1                            SOFT-DIRTY PTEs
2
3  The soft-dirty is a bit on a PTE which helps to track which pages a task
4writes to. In order to do this tracking one should
5
6  1. Clear soft-dirty bits from the task's PTEs.
7
8     This is done by writing "4" into the /proc/PID/clear_refs file of the
9     task in question.
10
11  2. Wait some time.
12
13  3. Read soft-dirty bits from the PTEs.
14
15     This is done by reading from the /proc/PID/pagemap. The bit 55 of the
16     64-bit qword is the soft-dirty one. If set, the respective PTE was
17     written to since step 1.
18
19
20  Internally, to do this tracking, the writable bit is cleared from PTEs
21when the soft-dirty bit is cleared. So, after this, when the task tries to
22modify a page at some virtual address the #PF occurs and the kernel sets
23the soft-dirty bit on the respective PTE.
24
25  Note, that although all the task's address space is marked as r/o after the
26soft-dirty bits clear, the #PF-s that occur after that are processed fast.
27This is so, since the pages are still mapped to physical memory, and thus all
28the kernel does is finds this fact out and puts both writable and soft-dirty
29bits on the PTE.
30
31  While in most cases tracking memory changes by #PF-s is more than enough
32there is still a scenario when we can lose soft dirty bits -- a task
33unmaps a previously mapped memory region and then maps a new one at exactly
34the same place. When unmap is called, the kernel internally clears PTE values
35including soft dirty bits. To notify user space application about such
36memory region renewal the kernel always marks new memory regions (and
37expanded regions) as soft dirty.
38
39  This feature is actively used by the checkpoint-restore project. You
40can find more details about it on http://criu.org
41
42
43-- Pavel Emelyanov, Apr 9, 2013
44