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

David Hildenbrand
2023-08-01T12:22:21Z

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?

--
Cheers,

David / dhildenb