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

[Cyrill Gorcunov <gorcunov@xxxxxxxxx>]

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.

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