summaryrefslogtreecommitdiff
path: root/Documentation/vm
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/vm')
-rw-r--r--Documentation/vm/soft-dirty.txt36
1 files changed, 36 insertions, 0 deletions
diff --git a/Documentation/vm/soft-dirty.txt b/Documentation/vm/soft-dirty.txt
new file mode 100644
index 000000000000..9a12a5956bc0
--- /dev/null
+++ b/Documentation/vm/soft-dirty.txt
@@ -0,0 +1,36 @@
+ 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.
+
+
+ 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