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- .. _soft_dirty:
- ===============
- Soft-Dirty PTEs
- ===============
- The soft-dirty is a bit on a PTE which helps to track which pages a task
- writes to. In order to do this tracking one should
- 1. Clear soft-dirty bits from the task's PTEs.
- This is done by writing "4" into the ``/proc/PID/clear_refs`` file of the
- task in question.
- 2. Wait some time.
- 3. Read soft-dirty bits from the PTEs.
- This is done by reading from the ``/proc/PID/pagemap``. The bit 55 of the
- 64-bit qword is the soft-dirty one. If set, the respective PTE was
- written to since step 1.
- Internally, to do this tracking, the writable bit is cleared from PTEs
- when the soft-dirty bit is cleared. So, after this, when the task tries to
- modify a page at some virtual address the #PF occurs and the kernel sets
- the soft-dirty bit on the respective PTE.
- Note, that although all the task's address space is marked as r/o after the
- soft-dirty bits clear, the #PF-s that occur after that are processed fast.
- This is so, since the pages are still mapped to physical memory, and thus all
- 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 lose soft dirty bits -- a task
- unmaps a previously mapped memory region and then maps a new one at exactly
- the same place. When unmap is called, the kernel internally clears PTE values
- including soft dirty bits. To notify user space application about such
- memory region renewal the kernel always marks new memory regions (and
- expanded regions) as soft dirty.
- This feature is actively used by the checkpoint-restore project. You
- can find more details about it on http://criu.org
- -- Pavel Emelyanov, Apr 9, 2013
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