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

[Jason Gunthorpe <jgg@nvidia.com>]

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.

If you do not fault in the zero pages to the PTEs then you may not get
invalidate callbacks.

Jason