[CRIU] Soft dirty pages in expanded VMA

[Diyu Zhou]

Hey Mike,

Thanks for your reply. I understand what you wrote. However, what I observe
is
that some of the "clean" pages are copied by the CRIU.

For example, let's say CRIU performs iterative checkpoints (let's say using
predump) on a dumpee process. After one checkpoint is completed, the dumpee
process calls sbrk to increase the size of the heap. What I observe is that
even the dumpee program didn't touch any pages in the heap, all the present
pages in the heap will be marked as dirty and copied in the next checkpoint
operation. Is there an explanation for such a behavior?

Thanks in advance for your help.

Best,
Diyu



On Tue, May 14, 2019 at 9:04 AM Mike Rapoport <rppt at linux.ibm.com> wrote:

> On Mon, May 13, 2019 at 04:10:54PM -0700, Diyu Zhou wrote:
> > Hey,
> >
> > Firstly, thanks for the great tool! It is very very helpful. I can see
> the
> > development of such a tool
> > is very challenging and you guys do several clever tricks to make it
> possible.
> > I enjoy a lot using
> > the tool as well as reading the source code and have learned a lot from
> it.
> >
> > I have a question regarding the implementation of the soft dirty pages.
> From
> > what I can tell, if
> > a VMA is expanded, all the pages within it will be marked as soft dirty.
> This
> > seems to me will
> > result in unnecessarily transferring clean pages when iterative dumps are
> > performed.
> > Is there any reason not just mark the pages in the expanded part of the
> VMA
> > soft dirty?
>
> When a VMA is expanded is is marked soft-dirty to ensure it would be
> scanned for the dirty pages.
> Each page has its own marking that says whether the page is dirty or not.
> The per-page information is available at /proc/PID/pagemap and CRIU uses it
> to actually decide what page should be dumped.
>
> > Thanks again for the wonderful tool and your help!
> >
> > Best,
> > Diyu
> >
> >
> >
>
> > _______________________________________________
> > CRIU mailing list
> > CRIU at openvz.org
> > https://lists.openvz.org/mailman/listinfo/criu
>
>
> --
> Sincerely yours,
> Mike.
>
>
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