PROBLEM: userfaultfd REGISTER minor mode on MAP_PRIVATE range fails

["David P. Reed" <dpreed@deepplum.com>]

[1.] One line summary of the problem: userfaultfd REGISTER minor mode on MAP_PRIVATE fails
[2.] Full description of the problem/report:
The userfaultfd man page and the kernel docs seem to indicate that an area mapped
MAP_PRIVATE|MAP_ANONYMOUS can be registered to handle MINOR page faults on regular pages.
However, testing showed that not to work. MAP_SHARED does allow registration for MINOR
page fault events, though.
Either the documentation or the code should be fixed, IMO. Now reading the code that rejects
this case in the kernel source, the test in vma_can_userfault() that rejects this is this
line: 
	if ((vm_flags & VM_UFFD_MINOR) &&
	    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
		return false;
which probably should include !vma_is_anonymous(vma).

Or maybe the COW that might happen if the program were forked is something that can't be handled, which seems odd.


[3.] Keywords (i.e., modules, networking, kernel): kernel, memory management
[4.] Kernel information
[4.1.] Kernel version (from /proc/version): Linux version 6.15.10-200.fc42.x86_64 (mockbuild@14a33d64645143cab3659d1335d9f80c) (gcc (GCC) 15.2.1 20250808 (Red Hat 15.2.1-1), GNU ld version 2.44-6.fc42) #1 SMP PREEMPT_DYNAMIC Fri Aug 15 15:57:06 UTC 2025


I can construct a program that exhibits just this bug, if that will help. For now, though, I have just replaced MAP_PRIVATE with MAP_SHARED on my anonymous pages, as a workaround. I don't fork, so there's no need for COW.