soft-dirty.rst (1801B)
1.. _soft_dirty: 2 3=============== 4Soft-Dirty PTEs 5=============== 6 7The soft-dirty is a bit on a PTE which helps to track which pages a task 8writes to. In order to do this tracking one should 9 10 1. Clear soft-dirty bits from the task's PTEs. 11 12 This is done by writing "4" into the ``/proc/PID/clear_refs`` file of the 13 task in question. 14 15 2. Wait some time. 16 17 3. Read soft-dirty bits from the PTEs. 18 19 This is done by reading from the ``/proc/PID/pagemap``. The bit 55 of the 20 64-bit qword is the soft-dirty one. If set, the respective PTE was 21 written to since step 1. 22 23 24Internally, to do this tracking, the writable bit is cleared from PTEs 25when the soft-dirty bit is cleared. So, after this, when the task tries to 26modify a page at some virtual address the #PF occurs and the kernel sets 27the soft-dirty bit on the respective PTE. 28 29Note, that although all the task's address space is marked as r/o after the 30soft-dirty bits clear, the #PF-s that occur after that are processed fast. 31This is so, since the pages are still mapped to physical memory, and thus all 32the kernel does is finds this fact out and puts both writable and soft-dirty 33bits on the PTE. 34 35While in most cases tracking memory changes by #PF-s is more than enough 36there is still a scenario when we can lose soft dirty bits -- a task 37unmaps a previously mapped memory region and then maps a new one at exactly 38the same place. When unmap is called, the kernel internally clears PTE values 39including soft dirty bits. To notify user space application about such 40memory region renewal the kernel always marks new memory regions (and 41expanded regions) as soft dirty. 42 43This feature is actively used by the checkpoint-restore project. You 44can find more details about it on http://criu.org 45 46 47-- Pavel Emelyanov, Apr 9, 2013