Revision d033ed9eeafc3bf33ce2de286ea2fb2c63e1c183 authored by Linus Torvalds on 03 July 2015, 21:52:25 UTC, committed by Linus Torvalds on 03 July 2015, 21:52:25 UTC
Pull hwspinlock updates from Ohad Ben-Cohen: - hwspinlock core DT support from Suman Anna - OMAP hwspinlock DT support from Suman Anna - QCOM hwspinlock DT support from Bjorn Andersson - a new CSR atlas7 hwspinlock driver from Wei Chen - CSR atlas7 hwspinlock DT binding document from Wei Chen - a tiny QCOM hwspinlock driver fix from Bjorn Andersson * tag 'hwspinlock-4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/ohad/hwspinlock: hwspinlock: qcom: Correct msb in regmap_field DT: hwspinlock: add the CSR atlas7 hwspinlock bindings document hwspinlock: add a CSR atlas7 driver hwspinlock: qcom: Add support for Qualcomm HW Mutex block DT: hwspinlock: Add binding documentation for Qualcomm hwmutex hwspinlock/omap: add support for dt nodes Documentation: dt: add the omap hwspinlock bindings document hwspinlock/core: add device tree support Documentation: dt: add common bindings for hwspinlock
soft-dirty.txt
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
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