From 1434a65ea625c51317ccdf06dabf4bd27d20fa10 Mon Sep 17 00:00:00 2001
From: Chengguang Xu <cgxu519@mykernel.net>
Date: Tue, 26 May 2020 09:35:57 +0800
Subject: [PATCH] ovl: drop negative dentry in upper layer

Negative dentries of upper layer are useless after construction of
overlayfs' own dentry and may keep in the memory long time even after
unmount of overlayfs instance. This patch tries to drop unnecessary
negative dentry of upper layer to effectively reclaim memory.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/overlayfs/namei.c | 35 ++++++++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index f9640d27ddab..5dc19dcea5a4 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -191,16 +191,36 @@ static bool ovl_is_opaquedir(struct dentry *dentry)
 	return ovl_check_dir_xattr(dentry, OVL_XATTR_OPAQUE);
 }
 
+static struct dentry *ovl_lookup_positive_unlocked(const char *name,
+						   struct dentry *base, int len,
+						   bool drop_negative)
+{
+	struct dentry *ret = lookup_one_len_unlocked(name, base, len);
+
+	if (!IS_ERR(ret) && d_flags_negative(smp_load_acquire(&ret->d_flags))) {
+		if (drop_negative && ret->d_lockref.count == 1) {
+			spin_lock(&ret->d_lock);
+			/* Recheck condition under lock */
+			if (d_is_negative(ret) && ret->d_lockref.count == 1)
+				__d_drop(ret);
+			spin_unlock(&ret->d_lock);
+		}
+		dput(ret);
+		ret = ERR_PTR(-ENOENT);
+	}
+	return ret;
+}
+
 static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 			     const char *name, unsigned int namelen,
 			     size_t prelen, const char *post,
-			     struct dentry **ret)
+			     struct dentry **ret, bool drop_negative)
 {
 	struct dentry *this;
 	int err;
 	bool last_element = !post[0];
 
-	this = lookup_positive_unlocked(name, base, namelen);
+	this = ovl_lookup_positive_unlocked(name, base, namelen, drop_negative);
 	if (IS_ERR(this)) {
 		err = PTR_ERR(this);
 		this = NULL;
@@ -276,7 +296,7 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 }
 
 static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
-			    struct dentry **ret)
+			    struct dentry **ret, bool drop_negative)
 {
 	/* Counting down from the end, since the prefix can change */
 	size_t rem = d->name.len - 1;
@@ -285,7 +305,7 @@ static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 
 	if (d->name.name[0] != '/')
 		return ovl_lookup_single(base, d, d->name.name, d->name.len,
-					 0, "", ret);
+					 0, "", ret, drop_negative);
 
 	while (!IS_ERR_OR_NULL(base) && d_can_lookup(base)) {
 		const char *s = d->name.name + d->name.len - rem;
@@ -298,7 +318,8 @@ static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 			return -EIO;
 
 		err = ovl_lookup_single(base, d, s, thislen,
-					d->name.len - rem, next, &base);
+					d->name.len - rem, next, &base,
+					drop_negative);
 		dput(dentry);
 		if (err)
 			return err;
@@ -830,7 +851,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	old_cred = ovl_override_creds(dentry->d_sb);
 	upperdir = ovl_dentry_upper(dentry->d_parent);
 	if (upperdir) {
-		err = ovl_lookup_layer(upperdir, &d, &upperdentry);
+		err = ovl_lookup_layer(upperdir, &d, &upperdentry, true);
 		if (err)
 			goto out;
 
@@ -888,7 +909,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		else
 			d.last = lower.layer->idx == roe->numlower;
 
-		err = ovl_lookup_layer(lower.dentry, &d, &this);
+		err = ovl_lookup_layer(lower.dentry, &d, &this, false);
 		if (err)
 			goto out_put;