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Diffstat (limited to 'Documentation/vm/soft-dirty.txt')
| -rw-r--r-- | Documentation/vm/soft-dirty.txt | 43 |
1 files changed, 43 insertions, 0 deletions
diff --git a/Documentation/vm/soft-dirty.txt b/Documentation/vm/soft-dirty.txt new file mode 100644 index 000000000..55684d11a --- /dev/null +++ b/Documentation/vm/soft-dirty.txt @@ -0,0 +1,43 @@ + 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 |