Re: Change soft-dirty interface?

[Minchan Kim <minchan@kernel.org>]

On Wed, Jun 19, 2013 at 01:31:28PM +0400, Pavel Emelyanov wrote:
> >>> Maybe do you have a concern about live-lock?
> >>
> >> No, I worry about potential races with which we or application can skip
> >> dirty page. Let me describe how CRIU uses existing soft-dirty implementation.
> >>
> >> 1. stop the task we want to work on
> >> 2. read the /proc/pid/pagemap file to find out which pages to
> >>    read. Those with soft-dirty _cleared_ should be _skipped_
> >> 3. read task's memory at calculated bitmap
> >> 4. reset soft dirty bits on task
> >> 5. resume task execution
> > 
> > Let me try to parse as my term.
> > 
> > 1. admin does "echo 4 > /proc/<target>/clear_refs"
> > 2. admin stop the target
> > 3. admin reads the /proc/<target>/pagemap and make bitmap
> >    with only soft-dirty marked pages so we can avoid unnecessary
> >    migration
> > 4. admin reads target's dirtied pages via bitmap from 3
> > 5. admin does "echo 4 > /proc/<target>/clear_refs" again to find
> >    future diry pages of the target.
> > 6. admin resumes the target
> > 
> > Right?
> 
> Almost, the step #1 looks excessive. We shouldn't clear the soft dirty
> _before_ stopping the target, otherwise we lose all the bits "collected"
> before it.
> 
> > If so, my interface is following as
> > 
> > 1. admin does set_softdirty(target, 0, 0, &token);
> >    (set_softdirty clears all soft-dirty bit from target process's
> >    page table.
> > 2. admin stop the target
> > 3. admin reads the /proc/target/pagemap and make bitmap
> >    with only soft-dirty marked pages so we can avoid unnecessary
> >    migration. 
> > 4. admins does get_softdirty(target, 0, 0, token) to confirm
> >    someone else spoiled since 1
> > 4-1. If it is reports error, then admins discard the bitmap got
> >      from 3 and have to read all memory.
> > 5. admin does set_softdirty(target, 0, 0, &token) again to find
> >    future dirty pages of the target  
> > 5. admin resumes the target.
> 
> Same here -- if we skip step #1, then we can merge steps 4 and 5 into
> one system call. Can we?
> 
> >>
> >> With the interface you propose the sequence presumably should look like
> >>
> >> 1. stop the task we want to work on
> >> 2. call set_softdirty + get_softdirty to get the soft-dirty bitmap and
> >>    reset one. If it reports error, then the soft-dirty we did before is
> >>    spoiled and all memory should be read (iow -- bitmap should be filled
> >>    with 1-s)
> >> 3. read task's memory at calculated bitmap
> >> 4. resume task execution
> > 
> >>
> >> Am I right with this? If yes, why do we need two calls, wouldn't it be better
> > 
> > I failed to parse your terms so I wrote scnario as my understanding
> > so please see my above sequence and if you have a comment, please ask
> > again.
> > 
> >> to merge them into one?
> > 
> > It's not hard part but I wanted to show my intention clearly.
> > If we all agree on, let's think over interface again.
> 
> For me the interface with a single syscall looks OK. If nobody else objects,
> I think you can go on with the kernel patches :) Presumably you can even
> use the criu project sources and tests to check how memory changes tracking
> works with the new interface.

Thanks for the good discussion!

-- 
Kind regards,
Minchan Kim

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