[SOLVED] Does userfaultfd now support file backed map?

Issue

I saw from the documentation of userfaultfd

https://manpages.debian.org/testing/manpages-dev/userfaultfd.2.en.html http://man7.org/linux/man-pages/man2/ioctl_userfaultfd.2.html

that userfaultfd will start supporting shared map since kernel 4.11. However, the documentation still looks very ambiguous in the sense that I'm still wondering will these include supporting file-backed mmap (which can also be MAP_SHARED)?

Solution

To answer definitively, since the information is not in the manual page(s), we can see the source.

Attempting to register a userfaultfd object with an address range must check whether that range is "compatible":

…
/* check not compatible vmas */
ret = -EINVAL;
if (!vma_can_userfault(cur))
    goto out_unlock;

and the definition of compatibility is:

if ((vm_flags & VM_UFFD_MINOR) &&
    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
    return false;
#ifndef CONFIG_PTE_MARKER_UFFD_WP
/*
 * If user requested uffd-wp but not enabled pte markers for
 * uffd-wp, then shmem & hugetlbfs are not supported but only
 * anonymous.
 */
if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma))
    return false;
#endif
return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
    vma_is_shmem(vma);

Thus, anonymous mappings or mappings in:

• tmpfs-backed, aka shared memory virtual filesystem (tmpfs, shmget) mappings are compatible
• when CONFIG_SHMEM is disabled, file-backed ranges which have been remapped (with generic_file_mmap) as shared are also compatible

The meaning of VM_UFFD_MINOR is support for notifications on minor page faults.

Answered By - Michael Foukarakis

Answer Checked By - Terry (WPSolving Volunteer)