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diff --git a/Documentation/vm/soft-dirty.rst b/Documentation/vm/soft-dirty.rst deleted file mode 100644 index cb0cfd6672fa..000000000000 --- a/Documentation/vm/soft-dirty.rst +++ /dev/null @@ -1,47 +0,0 @@ -.. _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 |