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.

[colin.king@canonical.com: fix several spelling mistakes]
  Link: http://lkml.kernel.org/r/20200420084241.65433-1-colin.king@canonical.com
[aneesh.kumar@linux.ibm.com: replace the usage of system(3) in the test]
  Link: http://lkml.kernel.org/r/20200429110727.89388-1-aneesh.kumar@linux.ibm.com
[kirill@shutemov.name: fixup for issues I've noticed]
  Link: http://lkml.kernel.org/r/20200429124816.jp272trghrzxx5j5@box
[jhubbard@nvidia.com: add khugepaged to .gitignore]
  Link: http://lkml.kernel.org/r/20200517002509.362401-1-jhubbard@nvidia.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Link: http://lkml.kernel.org/r/20200416160026.16538-1-kirill.shutemov@linux.intel.com
Link: http://lkml.kernel.org/r/20200416160026.16538-2-kirill.shutemov@linux.intel.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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