Add a design doc.

Signed-off-by: Yu Zhao <redacted>
Acked-by: Brian Geffon <redacted>
Acked-by: Jan Alexander Steffens (heftig) <redacted>
Acked-by: Oleksandr Natalenko <redacted>
Acked-by: Steven Barrett <redacted>
Acked-by: Suleiman Souhlal <redacted>
Tested-by: Daniel Byrne <redacted>
Tested-by: Donald Carr <redacted>
Tested-by: Holger Hoffstätte <redacted>
Tested-by: Konstantin Kharlamov <redacted>
Tested-by: Shuang Zhai <redacted>
Tested-by: Sofia Trinh <redacted>
Tested-by: Vaibhav Jain <redacted>
---
 Documentation/vm/index.rst        |   1 +
 Documentation/vm/multigen_lru.rst | 156 ++++++++++++++++++++++++++++++
 2 files changed, 157 insertions(+)
 create mode 100644 Documentation/vm/multigen_lru.rst

diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst
index 44365c4574a3..b48434300226 100644
--- a/Documentation/vm/index.rst
+++ b/Documentation/vm/index.rst

@@ -25,6 +25,7 @@ algorithms.  If you are looking for advice on simply allocating memory, see the
    ksm
    memory-model
    mmu_notifier
+   multigen_lru
    numa
    overcommit-accounting
    page_migration

diff --git a/Documentation/vm/multigen_lru.rst b/Documentation/vm/multigen_lru.rst
new file mode 100644
index 000000000000..cde60de16621
--- /dev/null
+++ b/Documentation/vm/multigen_lru.rst

@@ -0,0 +1,156 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+Multi-Gen LRU
+=============