Re: [Xen-devel] Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

[Minchan Kim <minchan@xxxxxxxxxx>]

Hello Cyrill,

On Thu, Aug 22, 2013 at 09:49:19AM +0400, Cyrill Gorcunov wrote:
> On Wed, Aug 21, 2013 at 04:42:53PM -0700, Andi Kleen wrote:
> > Cyrill Gorcunov <gorcunov@xxxxxxxxx> writes:
> > >
> > > Hi all, I worked on patch which would not touch PSE bit for dirty page
> > > tracking and the result is not that good:
> > >
> > >  - 2level pages now always page dirty if page is swapped in and out, 
> > > because
> > >    there is no space left in PTE (other than PSE bit)
> > 
> > Maybe just don't support soft dirty for 2 level page tables?
> > 
> > 2 level page tables should be really on the way out anyways, as they
> > have severe limits and do not support NX. With 3 levels there is enough
> > space.
> 
> Look, good thing is that 7th bit also available on the 4level pages
> (ie x86-64) without additional code modification, that's why I picked
> it in first place. I prepared the patch locally which doesn't use
> pse bit for tracking but it only makes code more complex.

I'm not sure you read this thread.
http://comments.gmane.org/gmane.linux.kernel.mm/101756
In summary, I'd like to use it to track sub-ranges of some processes.

I already had a time to investigate and it enhanced our workload x2 on ARM
so I'd like to expand the concept for more general purpose.

For it, arch-specific stuff would be hurdle for port.
So, I support your non-arch-specific solutions.
Just FYI. Such future plan shouldn't force you.

Thanks.

-- 
Kind regards,
Minchan Kim

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