tools/testing/selftests: add mremap() shrink test for multiple VMAs

tools/testing/selftests: add mremap() shrink test for multiple VMAs

Patch series "tools/testing: expand mremap testing".

Expand our mremap() testing to further assert that behaviour is as
expected.

There is a poorly documented mremap() feature whereby it is possible to
mremap() multiple VMAs (even with gaps) when shrinking, as long as the
resultant shrunk range spans only a single VMA.

So we start by asserting this behaviour functions correctly both with an
in-place shrink and a shrink/move.

Next, we further test the newly introduced ability to mremap() multiple
VMAs when performing a MAP_FIXED move (that is without the size being
changed), firstly by asserting that MREMAP_DONTUNMAP has no bearing on
this behaviour.

Finally, we explicitly test that such moves, when splitting source VMAs,
function correctly.

This patch (of 3):

There is an apparently little-known feature of mremap() whereby, in stark
contrast to other modes (other than the recently introduced capacity to
move multiple VMAs), the input source range span multiple VMAs with gaps
between.

This is, when shrinking a VMA, whether moving it or not, and the shrink
would reduce the range to a single VMA - this is permitted, as the shrink
is actioned by an unmap.

This patch adds tests to assert that this behaves as expected.

Link: https://lkml.kernel.org/r/cover.1753119043.git.lorenzo.stoakes@oracle.com
Link: https://lkml.kernel.org/r/f08122893a26092a2bec6e69443e87f468ffdbed.1753119043.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>