[PATCH v4 09/11] mm, swap: add debugfs counters for vswap

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

* used: virtual swap slots (pages) currently allocated
* alloc_reject: cumulative pages that failed to get a vswap slot

Signed-off-by: Nhat Pham <[email protected]>
---
 mm/swapfile.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 517fa58f8022..3b10173cddc3 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>
@@ -142,6 +143,8 @@ static DEFINE_PER_CPU(struct percpu_vswap_cluster, percpu_vswap_cluster) = {
 	.lock = INIT_LOCAL_LOCK(),
 };
 
+static atomic_long_t vswap_alloc_reject = ATOMIC_LONG_INIT(0);
+
 static bool vswap_alloc(struct folio *folio);
 static void vswap_mark_cache_only(struct swap_cluster_info *ci,
 				  unsigned int ci_off);
@@ -1983,6 +1986,7 @@ static bool vswap_alloc(struct folio *folio)
 
 	this_cpu_write(percpu_vswap_cluster.offset[order], SWAP_ENTRY_INVALID);
 	local_unlock(&percpu_vswap_cluster.lock);
+	atomic_long_add(folio_nr_pages(folio), &vswap_alloc_reject);
 	return false;
 }
 
@@ -4795,9 +4799,25 @@ early_param("vswap", early_vswap);
 /* vswap does no IO on its own. */
 static const struct swap_ops vswap_ops = { };
 
+static int vswap_used_get(void *data, u64 *val)
+{
+	*val = swap_usage_in_pages(vswap_si);
+	return 0;
+}
+DEFINE_DEBUGFS_ATTRIBUTE(vswap_used_fops, vswap_used_get, NULL, "%llu\n");
+
+static int vswap_alloc_reject_get(void *data, u64 *val)
+{
+	*val = atomic_long_read(&vswap_alloc_reject);
+	return 0;
+}
+DEFINE_DEBUGFS_ATTRIBUTE(vswap_alloc_reject_fops, vswap_alloc_reject_get, NULL,
+			 "%llu\n");
+
 static int __init vswap_init(void)
 {
 	struct swap_info_struct *si;
+	struct dentry *root;
 	unsigned long maxpages;
 	int err;
 
@@ -4841,6 +4861,12 @@ static int __init vswap_init(void)
 	mutex_unlock(&swapon_mutex);
 
 	vswap_si = si;
+
+	root = debugfs_create_dir("vswap", NULL);
+	debugfs_create_file("used", 0444, root, NULL, &vswap_used_fops);
+	debugfs_create_file("alloc_reject", 0444, root, NULL,
+			    &vswap_alloc_reject_fops);
+
 	pr_info("vswap: created virtual swap device (%lu pages)\n", maxpages);
 
 	/* Last: everything above must be visible before routing starts. */
-- 
2.53.0-Meta