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

David Hildenbrand
2023-08-01T12:26:11Z

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.

--
Cheers,

David / dhildenb