[RFC PATCH 5/5] mm, swap: add debugfs counters for vswap

[Nhat Pham <nphamcs@gmail.com>]

Add /sys/kernel/debug/vswap/ with two counters:

- used: number of virtual swap slots currently allocated
- alloc_reject: cumulative count of failed vswap allocations

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/swapfile.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index be901fb741e5..3740ab764405 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -7,6 +7,7 @@
  */
 
 #include <linux/blkdev.h>
+#include <linux/debugfs.h>
 #include <linux/mm.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/task.h>
@@ -132,6 +133,9 @@ static DEFINE_PER_CPU(struct percpu_swap_cluster, percpu_swap_cluster) = {
 	.lock = INIT_LOCAL_LOCK(),
 };
 
+static atomic_t __maybe_unused vswap_used = ATOMIC_INIT(0);
+static atomic_t __maybe_unused vswap_alloc_reject = ATOMIC_INIT(0);
+
 #ifdef CONFIG_VSWAP
 struct percpu_vswap_cluster {
 	unsigned long offset[SWAP_NR_ORDERS];
@@ -1993,11 +1997,13 @@ static bool vswap_alloc(struct folio *folio)
 	if (folio_test_swapcache(folio)) {
 		/* alloc_swap_scan_cluster updated percpu offset already */
 		local_unlock(&percpu_vswap_cluster.lock);
+		atomic_add(folio_nr_pages(folio), &vswap_used);
 		return true;
 	}
 
 	this_cpu_write(percpu_vswap_cluster.offset[order], SWAP_ENTRY_INVALID);
 	local_unlock(&percpu_vswap_cluster.lock);
+	atomic_add(folio_nr_pages(folio), &vswap_alloc_reject);
 	return false;
 }
 #endif
@@ -2554,8 +2560,10 @@ void __swap_cluster_free_entries(struct swap_info_struct *si,
 
 	VM_WARN_ON(ci->count < nr_pages);
 
-	if (is_vswap)
+	if (is_vswap) {
 		vswap_release_backing(ci, ci_start, nr_pages);
+		atomic_sub(nr_pages, &vswap_used);
+	}
 
 	ci->count -= nr_pages;
 	do {
@@ -4793,6 +4801,7 @@ struct swap_info_struct *vswap_si;
 static int __init vswap_init(void)
 {
 	struct swap_info_struct *si;
+	struct dentry *root;
 	unsigned long maxpages;
 	int err;
 
@@ -4819,6 +4828,11 @@ static int __init vswap_init(void)
 	mutex_unlock(&swapon_mutex);
 
 	vswap_si = si;
+
+	root = debugfs_create_dir("vswap", NULL);
+	debugfs_create_atomic_t("used", 0444, root, &vswap_used);
+	debugfs_create_atomic_t("alloc_reject", 0444, root, &vswap_alloc_reject);
+
 	pr_info("vswap: created virtual swap device (%lu pages)\n", maxpages);
 	return 0;
 
-- 
2.53.0-Meta