mm/mremap: optimize the start addresses in move_page_tables()

Patch series "Optimize mremap during mutual alignment within PMD", v6.

This patchset optimizes the start addresses in move_page_tables() and
tests the changes.  It addresses a warning [1] that occurs due to a
downward, overlapping move on a mutually-aligned offset within a PMD
during exec.  By initiating the copy process at the PMD level when such
alignment is present, we can prevent this warning and speed up the copying
process at the same time.  Linus Torvalds suggested this idea.  Check the
individual patches for more details.  [1]
https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@dhcp22.suse.cz/


This patch (of 7):

Recently, we see reports [1] of a warning that triggers due to
move_page_tables() doing a downward and overlapping move on a
mutually-aligned offset within a PMD.  By mutual alignment, I mean the
source and destination addresses of the mremap are at the same offset
within a PMD.

This mutual alignment along with the fact that the move is downward is
sufficient to cause a warning related to having an allocated PMD that does
not have PTEs in it.

This warning will only trigger when there is mutual alignment in the move
operation.  A solution, as suggested by Linus Torvalds [2], is to initiate
the copy process at the PMD level whenever such alignment is present. 
Implementing this approach will not only prevent the warning from being
triggered, but it will also optimize the operation as this method should
enhance the speed of the copy process whenever there's a possibility to
start copying at the PMD level.

Some more points:
a. The optimization can be done only when both the source and
destination of the mremap do not have anything mapped below it up to a
PMD boundary. I add support to detect that.

b. #1 is not a problem for the call to move_page_tables() from exec.c as
nothing is expected to be mapped below the source. However, for
non-overlapping mutually aligned moves as triggered by mremap(2), I
added support for checking such cases.

c. I currently only optimize for PMD moves, in the future I/we can build
on this work and do PUD moves as well if there is a need for this. But I
want to take it one step at a time.

d. We need to be careful about mremap of ranges within the VMA itself.
For this purpose, I added checks to determine if the address after
alignment falls within its VMA itself.

[1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@dhcp22.suse.cz/
[2] https://lore.kernel.org/all/CAHk-=whd7msp8reJPfeGNyt0LiySMT0egExx3TVZSX3Ok6X=9g@mail.gmail.com/

Link: https://lkml.kernel.org/r/20230903151328.2981432-1-joel@joelfernandes.org
Link: https://lkml.kernel.org/r/20230903151328.2981432-2-joel@joelfernandes.org


Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
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>