[RFC PATCH 10/10] mm/swapfile: add sysfs interface for ghost swap extension

[Baoquan He <baoquan.he@linux.dev>]

Add per-device attributes under /sys/kernel/mm/ghost_swap/ghost_<N>/:
  path (RO) — dentry basename
  max  (RW) — current max pages; write to trigger swap_ghost_extend_max()

The ghost_kobj field ties each ghost device to its sysfs directory,
created at swapon and removed at swapoff.

Signed-off-by: Baoquan He <baoquan.he@linux.dev>
---
 include/linux/swap.h |   1 +
 mm/swapfile.c        | 111 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 112 insertions(+)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index b87688b7d4ba..d9ae2a9e33cc 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -283,6 +283,7 @@ struct swap_info_struct {
 	struct xarray redirect_xa;	/* ghost offset → phys swp_entry_t */
 	struct vm_struct *cluster_vm;	/* ghost: sparse vm_area for cluster_info */
 	char *ghost_name;		/* ghost: dentry name for sysfs */
+	struct kobject *ghost_kobj;	/* ghost: sysfs per-device directory */
 #endif
 };
 
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 8bf336678887..ede10539156a 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -31,6 +31,8 @@
 #include <linux/security.h>
 #include <linux/backing-dev.h>
 #include <linux/mutex.h>
+#include <linux/sysfs.h>
+#include <linux/kobject.h>
 #include <linux/capability.h>
 #include <linux/syscalls.h>
 #include <linux/memcontrol.h>
@@ -60,6 +62,8 @@ static void move_cluster(struct swap_info_struct *si,
 #define GHOST_MAX_CLUSTER_BYTES (64UL * 1024 * 1024)
 static int swap_ghost_extend_max(struct swap_info_struct *si,
 				 unsigned int new_maxpages);
+static int ghost_sysfs_register(struct swap_info_struct *si);
+static void ghost_sysfs_unregister(struct swap_info_struct *si);
 #endif
 
 /*
@@ -3250,6 +3254,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->swap_file = NULL;
 	maxpages = p->max;
 	if (p->flags & SWP_GHOST) {
+		ghost_sysfs_unregister(p);
 		p->max = 0;
 		cluster_info = NULL;
 	} else {
@@ -4010,6 +4015,13 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		(si->flags & SWP_AREA_DISCARD) ? "s" : "",
 		(si->flags & SWP_PAGE_DISCARD) ? "c" : "");
 
+	if (si->flags & SWP_GHOST) {
+		error = ghost_sysfs_register(si);
+		if (error)
+			pr_warn("swapon: ghost sysfs register failed: %d\n",
+				error);
+	}
+
 	mutex_unlock(&swapon_mutex);
 	atomic_inc(&proc_poll_event);
 	wake_up_interruptible(&proc_poll_wait);
@@ -4357,6 +4369,105 @@ static int swap_ghost_extend_max(struct swap_info_struct *si,
 	spin_unlock(&swap_lock);
 	return 0;
 }
+
+/* ─── Ghost swap sysfs interface ─── */
+
+static struct kobject *ghost_swap_kobj;
+
+static struct swap_info_struct *ghost_kobj_to_si(struct kobject *kobj)
+{
+	int type;
+
+	for (type = 0; type < MAX_SWAPFILES; type++) {
+		struct swap_info_struct *si = READ_ONCE(swap_info[type]);
+
+		if (si && si->ghost_kobj == kobj && (si->flags & SWP_USED))
+			return si;
+	}
+	return NULL;
+}
+
+#define GHOST_ATTR_RO(_n) \
+	static ssize_t _n##_show(struct kobject *k, struct kobj_attribute *a, char *b)
+#define GHOST_ATTR_WO(_n) \
+	static ssize_t _n##_store(struct kobject *k, struct kobj_attribute *a, \
+				  const char *b, size_t n)
+
+GHOST_ATTR_RO(path)
+{
+	struct swap_info_struct *si = ghost_kobj_to_si(k);
+
+	if (!si || !si->ghost_name)
+		return -ENODEV;
+	return sysfs_emit(b, "%s\n", si->ghost_name);
+}
+
+GHOST_ATTR_RO(max)
+{
+	struct swap_info_struct *si = ghost_kobj_to_si(k);
+
+	if (!si)
+		return -ENODEV;
+	return sysfs_emit(b, "%u\n", si->max);
+}
+
+GHOST_ATTR_WO(max)
+{
+	struct swap_info_struct *si = ghost_kobj_to_si(k);
+	unsigned long val;
+	int ret;
+
+	if (!si)
+		return -ENODEV;
+	ret = kstrtoul(b, 10, &val);
+	if (ret)
+		return ret;
+	if (mutex_lock_interruptible(&swapon_mutex))
+		return -ERESTARTSYS;
+	ret = swap_ghost_extend_max(si, (unsigned int)val);
+	mutex_unlock(&swapon_mutex);
+	return ret ? ret : n;
+}
+
+static struct kobj_attribute ghost_attr_path = __ATTR_RO(path);
+static struct kobj_attribute ghost_attr_max  = __ATTR_RW(max);
+static struct attribute *ghost_attrs[] = {
+	&ghost_attr_path.attr,
+	&ghost_attr_max.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(ghost);
+
+static int ghost_sysfs_register(struct swap_info_struct *si)
+{
+	char name[32];
+	int ret;
+
+	if (!ghost_swap_kobj) {
+		ghost_swap_kobj = kobject_create_and_add("ghost_swap", mm_kobj);
+		if (!ghost_swap_kobj)
+			return -ENOMEM;
+	}
+	snprintf(name, sizeof(name), "ghost_%d", si->type);
+	si->ghost_kobj = kobject_create_and_add(name, ghost_swap_kobj);
+	if (!si->ghost_kobj)
+		return -ENOMEM;
+	ret = sysfs_create_groups(si->ghost_kobj, ghost_groups);
+	if (ret) {
+		kobject_put(si->ghost_kobj);
+		si->ghost_kobj = NULL;
+	}
+	return ret;
+}
+
+static void ghost_sysfs_unregister(struct swap_info_struct *si)
+{
+	if (!si->ghost_kobj)
+		return;
+	sysfs_remove_groups(si->ghost_kobj, ghost_groups);
+	kobject_put(si->ghost_kobj);
+	si->ghost_kobj = NULL;
+}
 #endif /* CONFIG_ZSWAP */
 
 static int __init swapfile_init(void)
-- 
2.54.0