[RFC PATCH v1.1 7/9] 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().  Fix it by setting the totalram one byte in the
case.

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 | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 3bdbf4fbf7147..e3f3ee75a3d33 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2816,10 +2816,16 @@ static __kernel_ulong_t damos_get_node_mem_bp(
 	}
 
 	si_meminfo_node(&i, goal->nid);
-	if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP)
+	if (!i.totalram)
+		return 10000;
+	if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP) {
 		numerator = i.totalram - i.freeram;
-	else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
+	} else {
+		/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
+		if (i.totalram < i.freeram)
+			return 0;
 		numerator = i.freeram;
+	}
 	return mult_frac(numerator, 10000, i.totalram);
 }
 
-- 
2.47.3