Re: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages)

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On Tue, Aug 01, 2023 at 02:22:12PM +0200, David Hildenbrand wrote:
> On 01.08.23 14:19, Jason Gunthorpe wrote:
> > On Tue, Aug 01, 2023 at 05:32:38AM +0000, Kasireddy, Vivek wrote:
> > 
> > > > You get another invalidate because the memfd removes the zero pages
> > > > that hmm_range_fault installed in the PTEs before replacing them with
> > > > actual writable pages. Then you do the move, and another
> > > > hmm_range_fault, and basically the whole thing over again. Except this
> > > > time instead of returning zero pages it returns actual writable
> > > > page.
> > 
> > > Ok, when I tested earlier (by registering an invalidate callback) but without
> > > hmm_range_fault(), I did not find this additional invalidate getting triggered.
> > > Let me try with hmm_range_fault() and see if everything works as expected.
> > > Thank you for your help.
> > 
> > If you do not get an invalidate then there is a pretty serious bug in
> > the mm that needs fixing.
> > 
> > Anything hmm_range_fault() returns must be invalidated if the
> > underying CPU mapping changes for any reasons. Since hmm_range_fault()
> > will populate zero pages when reading from a hole in a memfd, it must
> > also get an invalidation when the zero pages are changed into writable
> > pages.
> 
> Can you point me at the code that returns that (shared) zero page?

It calls handle_mm_fault() - shouldn't that do it? Same as if the CPU
read faulted the page?

Jason 



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