mirror of
https://github.com/torvalds/linux.git
synced 2024-12-27 13:22:23 +00:00
1ad1335dc5
Several documents in Documentation/vm fit quite well into the "admin/user guide" category. The documents that don't overload the reader with lots of implementation details and provide coherent description of certain feature can be moved to Documentation/admin-guide/mm. Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
48 lines
1.8 KiB
ReStructuredText
48 lines
1.8 KiB
ReStructuredText
.. _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
|