1=============== 2Soft-Dirty PTEs 3=============== 4 5The soft-dirty is a bit on a PTE which helps to track which pages a task 6writes to. In order to do this tracking one should 7 8 1. Clear soft-dirty bits from the task's PTEs. 9 10 This is done by writing "4" into the ``/proc/PID/clear_refs`` file of the 11 task in question. 12 13 2. Wait some time. 14 15 3. Read soft-dirty bits from the PTEs. 16 17 This is done by reading from the ``/proc/PID/pagemap``. The bit 55 of the 18 64-bit qword is the soft-dirty one. If set, the respective PTE was 19 written to since step 1. 20 21 22Internally, to do this tracking, the writable bit is cleared from PTEs 23when the soft-dirty bit is cleared. So, after this, when the task tries to 24modify a page at some virtual address the #PF occurs and the kernel sets 25the soft-dirty bit on the respective PTE. 26 27Note, that although all the task's address space is marked as r/o after the 28soft-dirty bits clear, the #PF-s that occur after that are processed fast. 29This is so, since the pages are still mapped to physical memory, and thus all 30the kernel does is finds this fact out and puts both writable and soft-dirty 31bits on the PTE. 32 33While in most cases tracking memory changes by #PF-s is more than enough 34there is still a scenario when we can lose soft dirty bits -- a task 35unmaps a previously mapped memory region and then maps a new one at exactly 36the same place. When unmap is called, the kernel internally clears PTE values 37including soft dirty bits. To notify user space application about such 38memory region renewal the kernel always marks new memory regions (and 39expanded regions) as soft dirty. 40 41This feature is actively used by the checkpoint-restore project. You 42can find more details about it on http://criu.org 43 44 45-- Pavel Emelyanov, Apr 9, 2013 46