[RFC PATCH v1.2 6/8] mm/damon/core: handle extreme memory state in damon_get_node_mem_bp()

[SJ Park <sj@kernel.org>]

In an extreme and unlikely situation, si_meminfo_node() might let the
caller show zero total ram.  That could cause a divide by zero in
damon_get_node_mem_bp().  It could also show free memory larger than the
total memory.  This could cause underflow and make DAMOS temporarily
make unexpected behavior.  Thanks to safety guards in the auto-tuning
feedback loop, that should not be a real problem, though.  Fix the
problems by respectively returning 100% and 0% for used and free memory
queries in the corner cases.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260328133216.9697-1-sj@kernel.org

Fixes: 0e1c773b501f ("mm/damon/core: introduce damos quota goal metrics for memory node utilization")
Cc: <stable@vger.kernel.org> # 6.16.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index e2900d0c984c9..75d71ac09515e 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2816,6 +2816,13 @@ static __kernel_ulong_t damos_get_node_mem_bp(
 	}
 
 	si_meminfo_node(&i, goal->nid);
+	if (!i.totalram || i.totalram < i.freeram) {
+		if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP)
+			return 10000;
+		else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
+			return 0;
+	}
+
 	if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP)
 		numerator = i.totalram - i.freeram;
 	else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
-- 
2.47.3