khugepaged: add self test

Patch series "thp/khugepaged improvements and CoW semantics", v4.

The patchset adds khugepaged selftest (anon-THP only for now), expands
cases khugepaged can handle and switches anon-THP copy-on-write handling
to 4k.

This patch (of 8):

The test checks if khugepaged is able to recover huge page where we expect
to do so.  It only covers anon-THP for now.

Currently the test shows few failures.  They are going to be addressed by
the following patches.

[[email protected]: fix several spelling mistakes]
  Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: replace the usage of system(3) in the test]
  Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: fixup for issues I've noticed]
  Link: http://lkml.kernel.org/r/20200429124816.jp272trghrzxx5j5@box
[[email protected]: add khugepaged to .gitignore]
  Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Colin Ian King <[email protected]>
Signed-off-by: Aneesh Kumar K.V <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Zi Yan <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Acked-by: Yang Shi <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: William Kucharski <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Mike Kravetz <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

tools/testing/selftests/vm/.gitignore

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 hugepage-mmap
 hugepage-shm
+khugepaged
 map_hugetlb
 map_populate
 thuge-gen

tools/testing/selftests/vm/Makefile

@@ -20,6 +20,7 @@ TEST_GEN_FILES += on-fault-limit
 TEST_GEN_FILES += thuge-gen
 TEST_GEN_FILES += transhuge-stress
 TEST_GEN_FILES += userfaultfd
+TEST_GEN_FILES += khugepaged
 
 ifneq (,$(filter $(MACHINE),arm64 ia64 mips64 parisc64 ppc64 ppc64le riscv64 s390x sh64 sparc64 x86_64))
 TEST_GEN_FILES += va_128TBswitch