[PATCH RFC 7/9] mm/page_owner: add debugfs file 'swap_page_owner'

[Mauricio Faria de Oliveira <mfo@igalia.com>]

Add debugfs file /sys/kernel/debug/swap_page_owner to report the allocation
stack traces for pages in swap space (i.e., dump the XArray).

Now, it is possible to list the allocation stack traces both for pages in
memory and swap space, via /sys/kernel/debug/{page_owner,swap_page_owner}.

Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
 mm/page_owner.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 589aa1d7b9b6..e014eb7caf56 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -1103,6 +1103,77 @@ static const struct file_operations page_owner_fops = {
 	.llseek		= lseek_page_owner,
 };
 
+#ifdef CONFIG_SWAP_PAGE_OWNER
+static ssize_t print_swap_page_owner(char __user *buf, size_t count,
+				     swp_entry_t entry,
+				     struct swap_page_owner *spo)
+{
+	int ret;
+	char *kbuf;
+
+	count = min_t(size_t, count, PAGE_SIZE);
+	kbuf = kmalloc(count, GFP_KERNEL);
+	if (!kbuf)
+		return -ENOMEM;
+
+	ret = scnprintf(kbuf, count,
+			"Page allocated via pid %d, tgid %d (%s), ts %llu ns\n",
+			spo->pid, spo->tgid, spo->comm, spo->ts_nsec);
+
+	ret += scnprintf(kbuf + ret, count - ret,
+			 "SWP entry 0x%lx\n", entry.val);
+
+	ret += stack_depot_snprint(spo->handle, kbuf + ret, count - ret, 0);
+	if (ret >= count)
+		goto err;
+
+	ret += snprintf(kbuf + ret, count - ret, "\n");
+	if (ret >= count)
+		goto err;
+
+	if (copy_to_user(buf, kbuf, ret))
+		ret = -EFAULT;
+
+	kfree(kbuf);
+	return ret;
+
+err:
+	kfree(kbuf);
+	return -ENOMEM;
+}
+
+static ssize_t read_swap_page_owner(struct file *file, char __user *buf,
+				    size_t count, loff_t *ppos)
+{
+	swp_entry_t entry = { .val = *ppos };
+	void *spo;
+	XA_STATE(xa_state, &swap_page_owners, entry.val);
+
+	if (!static_branch_unlikely(&swap_page_owner_inited))
+		return -EINVAL;
+
+	xa_lock(&swap_page_owners);
+	xas_for_each(&xa_state, spo, ULONG_MAX) {
+
+		/* Resume after current index. */
+		entry.val = xa_state.xa_index;
+		*ppos = entry.val + 1;
+
+		xa_unlock(&swap_page_owners);
+		return print_swap_page_owner(buf, count, entry,
+					     (struct swap_page_owner *) spo);
+	}
+	xa_unlock(&swap_page_owners);
+
+	return 0;
+}
+
+static const struct file_operations swap_page_owner_fops = {
+	.read		= read_swap_page_owner,
+	.llseek		= lseek_page_owner,
+};
+#endif
+
 static void *stack_start(struct seq_file *m, loff_t *ppos)
 {
 	struct stack *stack;
@@ -1247,6 +1318,10 @@ static int __init pageowner_init(void)
 			    &page_owner_stack_fops);
 	debugfs_create_file("count_threshold", 0600, dir, NULL,
 			    &page_owner_threshold_fops);
+#ifdef CONFIG_SWAP_PAGE_OWNER
+	debugfs_create_file("swap_page_owner", 0400, NULL, NULL,
+			    &swap_page_owner_fops);
+#endif
 	return 0;
 }
 late_initcall(pageowner_init)
-- 
2.51.0