On Fri, Aug 05, 2022 at 07:55:38PM +0200, Paolo Bonzini wrote:

On 7/21/22 11:44, David Hildenbrand wrote:

quoted

Also, I*think*  you can place pages via userfaultfd into shmem. Not
sure if that would count "auto alloc", but it would certainly bypass
fallocate().

Yeah, userfaultfd_register would probably have to forbid this for
F_SEAL_AUTO_ALLOCATE vmas.  Maybe the memfile_node can be reused for this,
adding a new MEMFILE_F_NO_AUTO_ALLOCATE flags?  Then userfault_register
would do something like memfile_node_get_flags(vma->vm_file) and check the
result.

Then we need change userfault_register uAPI for a new property flag.
Userspace should still the decision-maker for this flag.

This means moving this patch later, after "mm: Introduce memfile_notifier".

Yes, it makes sense now.

Chao

Thanks,

Paolo