Docs/mm/damon/design: document the power-of-two limitation for addr_unit

The min_region_sz is set as max(DAMON_MIN_REGION_SZ / addr_unit, 1).
DAMON_MIN_REGION_SZ is the same to PAGE_SIZE, and addr_unit is what the
user can arbitrarily set.  Commit c80f46ac228b ("mm/damon/core: disallow
non-power of two min_region_sz") made min_region_sz to always be a power
of two.  Hence, addr_unit should be a power of two when it is smaller than
PAGE_SIZE.  While 'addr_unit' is a user-exposed parameter, the rule is not
documented.  This can confuse users.  Specifically, if the user sets
addr_unit as a value that is smaller than PAGE_SIZE and not a power of
two, the setup will explicitly fail.

Document the rule on the design document.  Usage documents reference the
design document for detail, so updating only the design document should
suffice.

Link: https://lkml.kernel.org/r/20260307194222.202075-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index dd64f5d7f3193a7658d71e4108d4f5cbab8acd5f..ac795f30519c5e34157b084d62ff8cd98afeb9fe 100644 (file)
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -150,6 +150,8 @@ address on the given address space.  Support of ``address unit`` parameter is
 up to each operations set implementation.  ``paddr`` is the only operations set
 implementation that supports the parameter.
 
+If the value is smaller than ``PAGE_SIZE``, only a power of two should be used.
+
 .. _damon_core_logic:
 
 Core Logics