selftests: vm: add KSM merging across nodes test

Add check_ksm_numa_merge() function to test that pages in different NUMA
nodes are being handled properly.  First, two duplicate pages are
allocated in two separate NUMA nodes using the libnuma library.  Since
there is one unique page in each node, with merge_across_nodes = 0, there
won't be any shared pages.  If merge_across_nodes is set to 1, the pages
will be treated as usual duplicate pages and will be merged.  If NUMA
config is not enabled or the number of NUMA nodes is less than two, then
the test is skipped.  The test is run as follows: ./ksm_tests -N

Link: https://lkml.kernel.org/r/071c17b5b04ebb0dfeba137acc495e5dd9d2a719.1626252248.git.zhansayabagdaulet@gmail.com


Signed-off-by: Zhansaya Bagdauletkyzy <zhansayabagdaulet@gmail.com>
Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>