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mm: soft-dirty bits for user memory changes tracking
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 PTEs ("echo 4 > /proc/PID/clear_refs) 2. Wait some time. 3. Read soft-dirty bits (55'th in /proc/PID/pagemap2 entries) To do this tracking, the writable bit is cleared from PTEs when the soft-dirty bit is. Thus, 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 back writable, dirty and soft-dirty bits on the PTE. Another thing to note, is that when mremap moves PTEs they are marked with soft-dirty as well, since from the user perspective mremap modifies the virtual memory at mremap's new address. Signed-off-by: Pavel Emelyanov <[email protected]> Cc: Matt Mackall <[email protected]> Cc: Xiao Guangrong <[email protected]> Cc: Glauber Costa <[email protected]> Cc: Marcelo Tosatti <[email protected]> Cc: KOSAKI Motohiro <[email protected]> Cc: Stephen Rothwell <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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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 | ||
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 | ||
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 | ||
64-bit qword is the soft-dirty one. If set, the respective PTE was | ||
written to since step 1. | ||
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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. | ||
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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. | ||
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This feature is actively used by the checkpoint-restore project. You | ||
can find more details about it on http://criu.org | ||
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-- Pavel Emelyanov, Apr 9, 2013 |
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