Messages in this thread | | | Date | Fri, 12 Apr 2013 17:14:03 +0400 | From | Pavel Emelyanov <> | Subject | Re: [PATCH 5/5] mm: Soft-dirty bits for user memory changes tracking |
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On 04/12/2013 01:24 AM, Andrew Morton wrote: > On Thu, 11 Apr 2013 15:30:00 +0400 Pavel Emelyanov <xemul@parallels.com> 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.
Thanks, Pavel
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