[RFC v2 19/43] mm: PKRAM: allow preserved memory to be freed from userspace

[Anthony Yznaga <anthony.yznaga@oracle.com>]

To free all space utilized for preserved memory, one can write 0 to
/sys/kernel/pkram. This will destroy all PKRAM nodes that are not
currently being read or written.

Originally-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
---
 mm/pkram.c | 39 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/mm/pkram.c b/mm/pkram.c
index dcf84ba785a7..8700fd77dc67 100644
--- a/mm/pkram.c
+++ b/mm/pkram.c
@@ -493,6 +493,32 @@ static void pkram_truncate_node(struct pkram_node *node)
 	node->obj_pfn = 0;
 }
 
+/*
+ * Free all nodes that are not under operation.
+ */
+static void pkram_truncate(void)
+{
+	struct page *page, *tmp;
+	struct pkram_node *node;
+	LIST_HEAD(dispose);
+
+	mutex_lock(&pkram_mutex);
+	list_for_each_entry_safe(page, tmp, &pkram_nodes, lru) {
+		node = page_address(page);
+		if (!(node->flags & PKRAM_ACCMODE_MASK))
+			list_move(&page->lru, &dispose);
+	}
+	mutex_unlock(&pkram_mutex);
+
+	while (!list_empty(&dispose)) {
+		page = list_first_entry(&dispose, struct page, lru);
+		list_del(&page->lru);
+		node = page_address(page);
+		pkram_truncate_node(node);
+		pkram_free_page(node);
+	}
+}
+
 static void pkram_add_link(struct pkram_link *link, struct pkram_data_stream *pds)
 {
 	__u64 link_pfn = page_to_pfn(virt_to_page(link));
@@ -1233,8 +1259,19 @@ static ssize_t show_pkram_sb_pfn(struct kobject *kobj,
 	return sprintf(buf, "%lx\n", pfn);
 }
 
+static ssize_t store_pkram_sb_pfn(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	int val;
+
+	if (kstrtoint(buf, 0, &val) || val)
+		return -EINVAL;
+	pkram_truncate();
+	return count;
+}
+
 static struct kobj_attribute pkram_sb_pfn_attr =
-	__ATTR(pkram, 0444, show_pkram_sb_pfn, NULL);
+	__ATTR(pkram, 0644, show_pkram_sb_pfn, store_pkram_sb_pfn);
 
 static struct attribute *pkram_attrs[] = {
 	&pkram_sb_pfn_attr.attr,
-- 
1.8.3.1