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
