On Tue, 20 Apr 2021, Axel Rasmussen wrote:

With this change, userspace can resolve a minor fault within a
shmem-backed area with a UFFDIO_CONTINUE ioctl. The semantics for this
match those for hugetlbfs - we look up the existing page in the page
cache, and install a PTE for it.

This commit introduces a new helper: mcopy_atomic_install_pte.

Why handle UFFDIO_CONTINUE for shmem in mm/userfaultfd.c, instead of in
shmem.c? The existing userfault implementation only relies on shmem.c
for VM_SHARED VMAs. However, minor fault handling / CONTINUE work just
fine for !VM_SHARED VMAs as well. We'd prefer to handle CONTINUE for
shmem in one place, regardless of shared/private (to reduce code
duplication).

Why add a new mcopy_atomic_install_pte helper? A problem we have with
continue is that shmem_mcopy_atomic_pte() and mcopy_atomic_pte() are
*close* to what we want, but not exactly. We do want to setup the PTEs
in a CONTINUE operation, but we don't want to e.g. allocate a new page,
charge it (e.g. to the shmem inode), manipulate various flags, etc. Also
we have the problem stated above: shmem_mcopy_atomic_pte() and
mcopy_atomic_pte() both handle one-half of the problem (shared /
private) continue cares about. So, introduce mcontinue_atomic_pte(), to
handle all of the shmem continue cases. Introduce the helper so it
doesn't duplicate code with mcopy_atomic_pte().

In a future commit, shmem_mcopy_atomic_pte() will also be modified to
use this new helper. However, since this is a bigger refactor, it seems
most clear to do it as a separate change.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

If this "03/10" had been numbered 04/10, I would have said
Acked-by: Hugh Dickins <hughd@google.com>

But I find this new ordering incomprehensible - I'm surprised that it
even builds this way around (if it does): this patch is so much about
what has been enabled in "04/10" (references to UFFDIO_CONTINUE shmem
VMAs etc).

Does Peter still think this way round is better? If he does, then we
shall have to compromise by asking you just to squash the two together.

---
 mm/userfaultfd.c | 172 ++++++++++++++++++++++++++++++++++-------------
 1 file changed, 127 insertions(+), 45 deletions(-)