[RFC PATCH v1.1 8/9] mm/damon/core: handle extreme memory state in get_node_memcg_used_bp()

[SJ Park]

In extreme unlikely situations, total memory might be zero.  In less
extreme but still very unlikely situations, lruvec_page_state() calls
might let the caller show used memory larger than total memory.  In the
two cases, damos_get_node_memcg_used_bp() could cause division by zero,
or return underflowed value, respectively.  Handle the cases by
returning 100% and 0% for the two cases, respectively.

This issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260329154813.47382-1-sj@xxxxxxxxxx

Fixes: b74a120bcf50 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.19.x
Signed-off-by: SJ Park <sj@xxxxxxxxxx>
---
 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 e3f3ee75a3d33..67ad1f07c29a4 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2862,10 +2862,16 @@ static unsigned long damos_get_node_memcg_used_bp(
 	mem_cgroup_put(memcg);
 
 	si_meminfo_node(&i, goal->nid);
-	if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
+	if (!i.totalram)
+		return 10000;
+	if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP) {
 		numerator = used_pages;
-	else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
+	} else {
+		/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
+		if (i.totalram < used_pages)
+			return 0;
 		numerator = i.totalram - used_pages;
+	}
 	return mult_frac(numerator, 10000, i.totalram);
 }
 
-- 
2.47.3