mm/damon/core: disallow non-power of two min_region_sz

DAMON core uses min_region_sz parameter value as the DAMON region alignment. The alignment is made using ALIGN() and ALIGN_DOWN(), which support only the power of two alignments. But DAMON core API callers can set min_region_sz to an arbitrary number. Users can also set it indirectly, using addr_unit.

When the alignment is not properly set, DAMON behavior becomes difficult to expect and understand, makes it effectively broken. It doesn't cause a kernel crash-like significant issue, though.

Fix the issue by disallowing min_region_sz input that is not a power of two. Add the check to damon_commit_ctx(), as all DAMON API callers who set min_region_sz uses the function.

This can be a sort of behavioral change, but it does not break users, for the following reasons. As the symptom is making DAMON effectively broken, it is not reasonable to believe there are real use cases of non-power of two min_region_sz. There is no known use case or issue reports from the setup, either.

In future, if we find real use cases of non-power of two alignments and we can support it with low enough overhead, we can consider moving the restriction. But, for now, simply disallowing the corner case should be good enough as a hot fix.

---
 mm/damon/core.c | 3 +++
 1 file changed, 3 insertions(+)

index 925908415a041..55cbf86476006 100644
@@ -1252,6 +1252,9 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
 {
 	int err;
 
+	if (!is_power_of_2(src->min_region_sz))
+		return -EINVAL;
+
 	err = damon_commit_schemes(dst, src);
 	if (err)
 		return err;

base-commit: 918647226184ca20e5f86ee7d4df3e34c7433799

2.47.3