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

Jason Gunthorpe
2023-08-01T12:26:45Z

On Tue, Aug 01, 2023 at 02:26:03PM +0200, David Hildenbrand wrote:
> On 01.08.23 14:23, Jason Gunthorpe wrote:
> > 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?
>
> To the best of my knowledge, the shared zeropage is only used in
> MAP_PRIVATE|MAP_AON mappings and in weird DAX mappings.
>
> If that changed, we have to fix FOLL_PIN|FOLL_LONGTERM for MAP_SHARED VMAs.
>
> If you read-fault on a memfd hole, you should get a proper "zeroed"
> pagecache page that effectively "filled that hole" -- so there is no file
> hole anymore.

Sounds fine then :)

Jason