v4.19.13 snapshot.
diff --git a/fs/drop_caches.c b/fs/drop_caches.c
new file mode 100644
index 0000000..8237701
--- /dev/null
+++ b/fs/drop_caches.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Implement the manual drop-all-pagecache function
+ */
+
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/writeback.h>
+#include <linux/sysctl.h>
+#include <linux/gfp.h>
+#include "internal.h"
+
+/* A global variable is a bit ugly, but it keeps the code simple */
+int sysctl_drop_caches;
+
+static void drop_pagecache_sb(struct super_block *sb, void *unused)
+{
+ struct inode *inode, *toput_inode = NULL;
+
+ spin_lock(&sb->s_inode_list_lock);
+ list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+ spin_lock(&inode->i_lock);
+ if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
+ (inode->i_mapping->nrpages == 0)) {
+ spin_unlock(&inode->i_lock);
+ continue;
+ }
+ __iget(inode);
+ spin_unlock(&inode->i_lock);
+ spin_unlock(&sb->s_inode_list_lock);
+
+ invalidate_mapping_pages(inode->i_mapping, 0, -1);
+ iput(toput_inode);
+ toput_inode = inode;
+
+ spin_lock(&sb->s_inode_list_lock);
+ }
+ spin_unlock(&sb->s_inode_list_lock);
+ iput(toput_inode);
+}
+
+int drop_caches_sysctl_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *length, loff_t *ppos)
+{
+ int ret;
+
+ ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
+ if (ret)
+ return ret;
+ if (write) {
+ static int stfu;
+
+ if (sysctl_drop_caches & 1) {
+ iterate_supers(drop_pagecache_sb, NULL);
+ count_vm_event(DROP_PAGECACHE);
+ }
+ if (sysctl_drop_caches & 2) {
+ drop_slab();
+ count_vm_event(DROP_SLAB);
+ }
+ if (!stfu) {
+ pr_info("%s (%d): drop_caches: %d\n",
+ current->comm, task_pid_nr(current),
+ sysctl_drop_caches);
+ }
+ stfu |= sysctl_drop_caches & 4;
+ }
+ return 0;
+}