Re: [PATCH V7 6/9] iommufd: pfn reader for file mappings

[Alexey Kardashevskiy <aik@amd.com>]

On 8/11/24 11:01, Alexey Kardashevskiy wrote:
> 
> 
> On 8/11/24 01:08, Jason Gunthorpe wrote:
>> On Thu, Nov 07, 2024 at 09:00:50PM +1100, Alexey Kardashevskiy wrote:
>>> I was initially pointed at this patchset as an example of how my 
>>> guest_memfd
>>> pinning of private memory (CoCo VM) should look like. And so far it's 
>>> been
>>> good except the actual pinning part - memfd_pin_folios() - expects 
>>> the file
>>> to be shmem or hugetlbfs and guest_memfd is neither (well, last 
>>> couple of
>>> months). Is there any plan to add those? If none, I will then, just 
>>> wanted
>>> to check. At the moment I do what KVM does which is calling
>>> filemap_grab_folio(). Thanks,
>>
>> I understood that the guestmemfd folks really don't want typical
>> pinning behavior, you'll probably have to negotiate with them on
>> exactly what the interface is?
> 
> Yup I will, just checking the plans here.
> 
>> I though KVM had its own private interface to guestmemfd?
> 
> At the moment it is essentially this:
> 
> folio = filemap_grab_folio(file_inode(file)->i_mapping, index);

Turns out the complete sequence is:

folio = filemap_grab_folio(file_inode(file)->i_mapping, index);
folio_add_pins(folio, 1);
folio_put(folio);

so then unpin_user_page_range_dirty_lock() => 
gup_fast_unpin_user_pages() => gup_put_folio() work properly.

Later on, hopefully, guest_memfd will developed ability to mmap() and 
then the existing iommufd mapping API will "just work", I suppose. I'll 
carry on the above for now. Thanks,



> 
> Thanks,
> 

-- 
Alexey