It is possible that the aligned address falls on no existing mapping,
however that does not mean that we can just align it down to that.  This
test verifies that the "vma->vm_start != addr_to_align" check in
can_align_down() prevents disastrous results if aligning down when source
and dest are mutually aligned within a PMD but the source/dest addresses
requested are not at the beginning of the respective mapping containing
these addresses.

Link: https://lkml.kernel.org/r/20230903151328.2981432-8-joel@joelfernandes.orgSigned-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>