Re: [PATCH 5/5] mm: Soft-dirty bits for user memory changes tracking

From: Pavel Emelyanov
Date: Fri Apr 12 2013 - 09:14:16 EST

On 04/12/2013 01:24 AM, Andrew Morton wrote:
> On Thu, 11 Apr 2013 15:30:00 +0400 Pavel Emelyanov <xemul@xxxxxxxxxxxxx> wrote:
>> 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.
>> ...
>> +config MEM_SOFT_DIRTY
>> + bool "Track memory changes"
>> + depends on CHECKPOINT_RESTORE && X86
> I guess we can add the CHECKPOINT_RESTORE dependency for now, but it is
> a general facility and I expect others will want to get their hands on
> it for unrelated things.

OK. Just tell me when you need the dependency removing patch.

>>From that perspective, the dependency on X86 is awful. What's the
> problem here and what do other architectures need to do to be able to
> support the feature?

The problem here is that I don't know what free bits are available on
page table entries on other architectures. I was about to resolve this
for ARM very soon, but for the rest of them I need help from other people.

> You have a test application, I assume. It would be helpful if we could
> get that into tools/testing/selftests.

If a very stupid 10-lines test is OK, then I can cook a patch with it.

Other than this I test this using the whole CRIU project, which is too
big for inclusion.

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