[RFC PATCH 2/2] ceph: quota: fix quota subdir mounts

[Luis Henriques]

The CephFS kernel client doesn't enforce quotas that are set in a
directory that isn't visible in the mount point.  For example, given the
path '/dir1/dir2', if quotas are set in 'dir1' and the mount is done in with

  mount -t ceph <server>:<port>:/dir1/ /mnt

then the client can't access the 'dir1' inode from the quota realm dir2
belongs to.

This patch fixes this by simply doing an MDS LOOKUPINO Op and grabbing a
reference to it (so that it doesn't disappear again).  This also requires an
extra field in ceph_snap_realm so that we know we have to release that
reference when destroying the realm.

Links: https://tracker.ceph.com/issues/3848
Reported-by: Hendrik Peyerl <hpeyerl@xxxxxxxxxxxx>
Signed-off-by: Luis Henriques <lhenriques@xxxxxxxx>
---
 fs/ceph/caps.c  |  2 +-
 fs/ceph/quota.c | 30 +++++++++++++++++++++++++++---
 fs/ceph/snap.c  |  3 +++
 fs/ceph/super.h |  2 ++
 4 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index bba28a5034ba..e79994ff53d6 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -1035,7 +1035,7 @@ static void drop_inode_snap_realm(struct ceph_inode_info *ci)
 	list_del_init(&ci->i_snap_realm_item);
 	ci->i_snap_realm_counter++;
 	ci->i_snap_realm = NULL;
-	if (realm->ino == ci->i_vino.ino)
+	if ((realm->ino == ci->i_vino.ino) && !realm->own_inode)
 		realm->inode = NULL;
 	spin_unlock(&realm->inodes_with_caps_lock);
 	ceph_put_snap_realm(ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc,
diff --git a/fs/ceph/quota.c b/fs/ceph/quota.c
index 9455d3aef0c3..f6b972d222e4 100644
--- a/fs/ceph/quota.c
+++ b/fs/ceph/quota.c
@@ -22,7 +22,16 @@ void ceph_adjust_quota_realms_count(struct inode *inode, bool inc)
 static inline bool ceph_has_realms_with_quotas(struct inode *inode)
 {
 	struct ceph_mds_client *mdsc = ceph_inode_to_client(inode)->mdsc;
-	return atomic64_read(&mdsc->quotarealms_count) > 0;
+	struct super_block *sb = mdsc->fsc->sb;
+
+	if (atomic64_read(&mdsc->quotarealms_count) > 0)
+		return true;
+	/* if root is the real CephFS root, we don't have quota realms */
+	if (sb->s_root->d_inode &&
+	    (sb->s_root->d_inode->i_ino == CEPH_INO_ROOT))
+		return false;
+	/* otherwise, we can't know for sure */
+	return true;
 }
 
 void ceph_handle_quota(struct ceph_mds_client *mdsc,
@@ -166,6 +175,7 @@ static bool check_quota_exceeded(struct inode *inode, enum quota_check_op op,
 		return false;
 
 	down_read(&mdsc->snap_rwsem);
+restart:
 	realm = ceph_inode(inode)->i_snap_realm;
 	if (realm)
 		ceph_get_snap_realm(mdsc, realm);
@@ -176,8 +186,22 @@ static bool check_quota_exceeded(struct inode *inode, enum quota_check_op op,
 		spin_lock(&realm->inodes_with_caps_lock);
 		in = realm->inode ? igrab(realm->inode) : NULL;
 		spin_unlock(&realm->inodes_with_caps_lock);
-		if (!in)
-			break;
+		if (!in) {
+			up_read(&mdsc->snap_rwsem);
+			in = ceph_lookup_inode(inode->i_sb, realm->ino);
+			down_read(&mdsc->snap_rwsem);
+			if (IS_ERR(in)) {
+				pr_warn("Can't lookup inode %llx (err: %ld)\n",
+					realm->ino, PTR_ERR(in));
+				break;
+			}
+			spin_lock(&realm->inodes_with_caps_lock);
+			realm->inode = in;
+			realm->own_inode = true;
+			spin_unlock(&realm->inodes_with_caps_lock);
+			ceph_put_snap_realm(mdsc, realm);
+			goto restart;
+		}
 
 		ci = ceph_inode(in);
 		spin_lock(&ci->i_ceph_lock);
diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
index f74193da0e09..c84ed8e8526a 100644
--- a/fs/ceph/snap.c
+++ b/fs/ceph/snap.c
@@ -117,6 +117,7 @@ static struct ceph_snap_realm *ceph_create_snap_realm(
 
 	atomic_set(&realm->nref, 1);    /* for caller */
 	realm->ino = ino;
+	realm->own_inode = false;
 	INIT_LIST_HEAD(&realm->children);
 	INIT_LIST_HEAD(&realm->child_item);
 	INIT_LIST_HEAD(&realm->empty_item);
@@ -184,6 +185,8 @@ static void __destroy_snap_realm(struct ceph_mds_client *mdsc,
 	kfree(realm->prior_parent_snaps);
 	kfree(realm->snaps);
 	ceph_put_snap_context(realm->cached_context);
+	if (realm->own_inode && realm->inode)
+		iput(realm->inode);
 	kfree(realm);
 }
 
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index ce51e98b08ec..3f0d74d2150f 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -764,6 +764,8 @@ struct ceph_snap_realm {
 	atomic_t nref;
 	struct rb_node node;
 
+	bool own_inode;     /* true if we hold a ref to the inode */
+
 	u64 created, seq;
 	u64 parent_ino;
 	u64 parent_since;   /* snapid when our current parent became so */