HAVE_MOVE_PMD enables remapping pages at the PMD level if both the
source and destination addresses are PMD-aligned.

HAVE_MOVE_PMD is already enabled on x86. The original patch [1] that
introduced this config did not enable it on arm64 at the time because
of performance issues with flushing the TLB on every PMD move. These
issues have since been addressed in more recent releases with
improvements to the arm64 TLB invalidation and core mmu_gather code as
Will Deacon mentioned in [2].

>From the data below, it can be inferred that there is approximately
8x improvement in performance when HAVE_MOVE_PMD is enabled on arm64.

--------- Test Results ----------

The following results were obtained on an arm64 device running a 5.4
kernel, by remapping a PMD-aligned, 1GB sized region to a PMD-aligned
destination. The results from 10 iterations of the test are given below.
All times are in nanoseconds.

Control    HAVE_MOVE_PMD

9220833    1247761
9002552    1219896
9254115    1094792
8725885    1227760
9308646    1043698
9001667    1101771
8793385    1159896
8774636    1143594
9553125    1025833
9374010    1078125

9100885.4  1134312.6    <-- Mean Time in nanoseconds

Total mremap time for a 1GB sized PMD-aligned region drops from
~9.1 milliseconds to ~1.1 milliseconds. (~8x speedup).

[1] https://lore.kernel.org/r/20181108181201.88826-3-joelaf@google.com
[2] https://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg140837.htmlSigned-off-by: NKalesh Singh <kaleshsingh@google.com>
Acked-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201014005320.2233162-3-kaleshsingh@google.com
Link: https://lore.kernel.org/kvmarm/20181029102840.GC13965@arm.com/Signed-off-by: NWill Deacon <will@kernel.org>