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diff --git a/Documentation/admin-guide/mm/soft-dirty.rst b/Documentation/admin-guide/mm/soft-dirty.rst new file mode 100644 index 0000000000..aeea936caa --- /dev/null +++ b/Documentation/admin-guide/mm/soft-dirty.rst @@ -0,0 +1,45 @@ +=============== +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 |