44 lines
1.7 KiB
Plaintext
44 lines
1.7 KiB
Plaintext
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SOFT-DIRTY PTEs
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The soft-dirty is a bit on a PTE which helps to track which pages a task
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writes to. In order to do this tracking one should
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1. Clear soft-dirty bits from the task's PTEs.
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This is done by writing "4" into the /proc/PID/clear_refs file of the
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task in question.
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2. Wait some time.
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3. Read soft-dirty bits from the PTEs.
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This is done by reading from the /proc/PID/pagemap. The bit 55 of the
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64-bit qword is the soft-dirty one. If set, the respective PTE was
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written to since step 1.
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Internally, to do this tracking, the writable bit is cleared from PTEs
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when the soft-dirty bit is cleared. So, after this, when the task tries to
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modify a page at some virtual address the #PF occurs and the kernel sets
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the soft-dirty bit on the respective PTE.
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Note, that although all the task's address space is marked as r/o after the
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soft-dirty bits clear, the #PF-s that occur after that are processed fast.
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This is so, since the pages are still mapped to physical memory, and thus all
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the kernel does is finds this fact out and puts both writable and soft-dirty
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bits on the PTE.
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While in most cases tracking memory changes by #PF-s is more than enough
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there is still a scenario when we can lose soft dirty bits -- a task
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unmaps a previously mapped memory region and then maps a new one at exactly
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the same place. When unmap is called, the kernel internally clears PTE values
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including soft dirty bits. To notify user space application about such
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memory region renewal the kernel always marks new memory regions (and
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expanded regions) as soft dirty.
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This feature is actively used by the checkpoint-restore project. You
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can find more details about it on http://criu.org
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-- Pavel Emelyanov, Apr 9, 2013
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