Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd

[Kirill A. Shutemov]

On Fri, Aug 05, 2022 at 07:55:38PM +0200, Paolo Bonzini wrote:
> On 7/21/22 11:44, David Hildenbrand wrote:
> > 
> > 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?

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov