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docs/vm: soft-dirty.txt: convert to ReST format 

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mike Rapoport 2018-03-21 21:22:38 +02:00 committed by Jonathan Corbet
parent 0c14398bf2
commit 0015190af2
1 changed files with 12 additions and 8 deletions
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20
Documentation/vm/soft-dirty.txt
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@ -1,34 +1,38 @@
SOFT-DIRTY PTEs .. _soft_dirty:
The soft-dirty is a bit on a PTE which helps to track which pages a task ===============
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 writes to. In order to do this tracking one should
1. Clear soft-dirty bits from the task's PTEs. 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 This is done by writing "4" into the ``/proc/PID/clear_refs`` file of the
task in question. task in question.
2. Wait some time. 2. Wait some time.
3. Read soft-dirty bits from the PTEs. 3. Read soft-dirty bits from the PTEs.
This is done by reading from the /proc/PID/pagemap. The bit 55 of the 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 64-bit qword is the soft-dirty one. If set, the respective PTE was
written to since step 1. written to since step 1.
Internally, to do this tracking, the writable bit is cleared from PTEs 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 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 modify a page at some virtual address the #PF occurs and the kernel sets
the soft-dirty bit on the respective PTE. the soft-dirty bit on the respective PTE.
Note, that although all the task's address space is marked as r/o after the 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. 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 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 the kernel does is finds this fact out and puts both writable and soft-dirty
bits on the PTE. bits on the PTE.
While in most cases tracking memory changes by #PF-s is more than enough 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 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 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 the same place. When unmap is called, the kernel internally clears PTE values
@ -36,7 +40,7 @@ including soft dirty bits. To notify user space application about such
memory region renewal the kernel always marks new memory regions (and memory region renewal the kernel always marks new memory regions (and
expanded regions) as soft dirty. expanded regions) as soft dirty.
This feature is actively used by the checkpoint-restore project. You This feature is actively used by the checkpoint-restore project. You
can find more details about it on http://criu.org can find more details about it on http://criu.org