On 18.08.22 01:41, Kirill A. Shutemov wrote:

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

quoted

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.

I donno, memory allocation with userfaultfd looks pretty intentional to
me. Why would F_SEAL_AUTO_ALLOCATE prevent it?

Can't we say the same about a write()?

Maybe we would need it in the future for post-copy migration or something?

Or existing practises around userfaultfd touch memory randomly and
therefore incompatible with F_SEAL_AUTO_ALLOCATE intent?

Note, that userfaultfd is only relevant for shared memory as it requires
VMA which we don't have for MFD_INACCESSIBLE.

This feature (F_SEAL_AUTO_ALLOCATE) is independent of all the lovely
encrypted VM stuff, so it doesn't matter how it relates to MFD_INACCESSIBLE.

-- 
Thanks,

David / dhildenb