userfaultfd/selftests: add test exercising minor fault handling

Fix a dormant bug in userfaultfd_events_test(), where we did `return
faulting_process(0)` instead of `exit(faulting_process(0))`.  This
caused the forked process to keep running, trying to execute any further
test cases after the events test in parallel with the "real" process.

Add a simple test case which exercises minor faults.  In short, it does
the following:

1. "Sets up" an area (area_dst) and a second shared mapping to the same
   underlying pages (area_dst_alias).

2. Register one of these areas with userfaultfd, in minor fault mode.

3. Start a second thread to handle any minor faults.

4. Populate the underlying pages with the non-UFFD-registered side of
   the mapping. Basically, memset() each page with some arbitrary
   contents.

5. Then, using the UFFD-registered mapping, read all of the page
   contents, asserting that the contents match expectations (we expect
   the minor fault handling thread can modify the page contents before
   resolving the fault).

The minor fault handling thread, upon receiving an event, flips all the
bits (~) in that page, just to prove that it can modify it in some
arbitrary way.  Then it issues a UFFDIO_CONTINUE ioctl, to setup the
mapping and resolve the fault.  The reading thread should wake up and
see this modification.

Currently the minor fault test is only enabled in hugetlb_shared mode,
as this is the only configuration the kernel feature supports.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Axel Rasmussen <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Cc: Adam Ruprecht <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Cannon Matthews <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Chinwen Chang <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: "Dr . David Alan Gilbert" <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: "Michal Koutn" <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Oliver Upton <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Shawn Anastasio <[email protected]>
Cc: Steven Price <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions