fs/9p: mitigate inode collisions

Detect and mitigate inode collsions that now occur since we
fixed 9p generating duplicate inode structures.  Underlying
cause of these appears to be a race condition between reuse
of inode numbers in underlying file system and cleanup of
inode numbers in the client.  Enabling caching
makes this much more likely to happen as it increases cleanup
latency due to writebacks.
Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>

fs/9p/v9fs.h

@@ -179,13 +179,14 @@ extern int v9fs_vfs_rename(struct mnt_idmap *idmap,
 			   struct inode *old_dir, struct dentry *old_dentry,
 			   struct inode *new_dir, struct dentry *new_dentry,
 			   unsigned int flags);
-extern struct inode *v9fs_fid_iget(struct super_block *sb, struct p9_fid *fid);
+extern struct inode *v9fs_fid_iget(struct super_block *sb, struct p9_fid *fid,
+						bool new);
 extern const struct inode_operations v9fs_dir_inode_operations_dotl;
 extern const struct inode_operations v9fs_file_inode_operations_dotl;
 extern const struct inode_operations v9fs_symlink_inode_operations_dotl;
 extern const struct netfs_request_ops v9fs_req_ops;
 extern struct inode *v9fs_fid_iget_dotl(struct super_block *sb,
-					struct p9_fid *fid);
+						struct p9_fid *fid, bool new);
 
 /* other default globals */
 #define V9FS_PORT	564
@@ -224,12 +225,12 @@ static inline int v9fs_proto_dotl(struct v9fs_session_info *v9ses)
  */
 static inline struct inode *
 v9fs_get_inode_from_fid(struct v9fs_session_info *v9ses, struct p9_fid *fid,
-			struct super_block *sb)
+			struct super_block *sb, bool new)
 {
 	if (v9fs_proto_dotl(v9ses))
-		return v9fs_fid_iget_dotl(sb, fid);
+		return v9fs_fid_iget_dotl(sb, fid, new);
 	else
-		return v9fs_fid_iget(sb, fid);
+		return v9fs_fid_iget(sb, fid, new);
 }
 
 #endif

fs/9p/vfs_inode.c

@@ -364,7 +364,8 @@ void v9fs_evict_inode(struct inode *inode)
 		clear_inode(inode);
 }
 
-struct inode *v9fs_fid_iget(struct super_block *sb, struct p9_fid *fid)
+struct inode *
+v9fs_fid_iget(struct super_block *sb, struct p9_fid *fid, bool new)
 {
 	dev_t rdev;
 	int retval;
@@ -376,8 +377,18 @@ struct inode *v9fs_fid_iget(struct super_block *sb, struct p9_fid *fid)
 	inode = iget_locked(sb, QID2INO(&fid->qid));
 	if (unlikely(!inode))
 		return ERR_PTR(-ENOMEM);
-	if (!(inode->i_state & I_NEW))
-		return inode;
+	if (!(inode->i_state & I_NEW)) {
+		if (!new) {
+			goto done;
+		} else {
+			p9_debug(P9_DEBUG_VFS, "WARNING: Inode collision %ld\n",
+						inode->i_ino);
+			iput(inode);
+			remove_inode_hash(inode);
+			inode = iget_locked(sb, QID2INO(&fid->qid));
+			WARN_ON(!(inode->i_state & I_NEW));
+		}
+	}
 
 	/*
 	 * initialize the inode with the stat info
@@ -401,11 +412,11 @@ struct inode *v9fs_fid_iget(struct super_block *sb, struct p9_fid *fid)
 	v9fs_set_netfs_context(inode);
 	v9fs_cache_inode_get_cookie(inode);
 	unlock_new_inode(inode);
+done:
 	return inode;
 error:
 	iget_failed(inode);
 	return ERR_PTR(retval);
-
 }
 
 /**
@@ -437,8 +448,15 @@ static int v9fs_at_to_dotl_flags(int flags)
  */
 static void v9fs_dec_count(struct inode *inode)
 {
-	if (!S_ISDIR(inode->i_mode) || inode->i_nlink > 2)
-		drop_nlink(inode);
+	if (!S_ISDIR(inode->i_mode) || inode->i_nlink > 2) {
+		if (inode->i_nlink) {
+			drop_nlink(inode);
+		} else {
+			p9_debug(P9_DEBUG_VFS,
+						"WARNING: unexpected i_nlink zero %d inode %ld\n",
+						inode->i_nlink, inode->i_ino);
+		}
+	}
 }
 
 /**
@@ -489,6 +507,9 @@ static int v9fs_remove(struct inode *dir, struct dentry *dentry, int flags)
 		} else
 			v9fs_dec_count(inode);
 
+		if (inode->i_nlink <= 0)	/* no more refs unhash it */
+			remove_inode_hash(inode);
+
 		v9fs_invalidate_inode_attr(inode);
 		v9fs_invalidate_inode_attr(dir);
 
@@ -554,7 +575,7 @@ v9fs_create(struct v9fs_session_info *v9ses, struct inode *dir,
 		/*
 		 * instantiate inode and assign the unopened fid to the dentry
 		 */
-		inode = v9fs_get_inode_from_fid(v9ses, fid, dir->i_sb);
+		inode = v9fs_get_inode_from_fid(v9ses, fid, dir->i_sb, true);
 		if (IS_ERR(inode)) {
 			err = PTR_ERR(inode);
 			p9_debug(P9_DEBUG_VFS,
@@ -683,7 +704,7 @@ struct dentry *v9fs_vfs_lookup(struct inode *dir, struct dentry *dentry,
 	else if (IS_ERR(fid))
 		inode = ERR_CAST(fid);
 	else
-		inode = v9fs_get_inode_from_fid(v9ses, fid, dir->i_sb);
+		inode = v9fs_get_inode_from_fid(v9ses, fid, dir->i_sb, false);
 	/*
 	 * If we had a rename on the server and a parallel lookup
 	 * for the new name, then make sure we instantiate with

fs/9p/vfs_inode_dotl.c

@@ -52,7 +52,10 @@ static kgid_t v9fs_get_fsgid_for_create(struct inode *dir_inode)
 	return current_fsgid();
 }
 
-struct inode *v9fs_fid_iget_dotl(struct super_block *sb, struct p9_fid *fid)
+
+
+struct inode *
+v9fs_fid_iget_dotl(struct super_block *sb, struct p9_fid *fid, bool new)
 {
 	int retval;
 	struct inode *inode;
@@ -62,8 +65,18 @@ struct inode *v9fs_fid_iget_dotl(struct super_block *sb, struct p9_fid *fid)
 	inode = iget_locked(sb, QID2INO(&fid->qid));
 	if (unlikely(!inode))
 		return ERR_PTR(-ENOMEM);
-	if (!(inode->i_state & I_NEW))
-		return inode;
+	if (!(inode->i_state & I_NEW)) {
+		if (!new) {
+			goto done;
+		} else { /* deal with race condition in inode number reuse */
+			p9_debug(P9_DEBUG_ERROR, "WARNING: Inode collision %lx\n",
+						inode->i_ino);
+			iput(inode);
+			remove_inode_hash(inode);
+			inode = iget_locked(sb, QID2INO(&fid->qid));
+			WARN_ON(!(inode->i_state & I_NEW));
+		}
+	}
 
 	/*
 	 * initialize the inode with the stat info
@@ -90,12 +103,11 @@ struct inode *v9fs_fid_iget_dotl(struct super_block *sb, struct p9_fid *fid)
 		goto error;
 
 	unlock_new_inode(inode);
-
+done:
 	return inode;
 error:
 	iget_failed(inode);
 	return ERR_PTR(retval);
-
 }
 
 struct dotl_openflag_map {
@@ -247,7 +259,7 @@ v9fs_vfs_atomic_open_dotl(struct inode *dir, struct dentry *dentry,
 		p9_debug(P9_DEBUG_VFS, "p9_client_walk failed %d\n", err);
 		goto out;
 	}
-	inode = v9fs_fid_iget_dotl(dir->i_sb, fid);
+	inode = v9fs_fid_iget_dotl(dir->i_sb, fid, true);
 	if (IS_ERR(inode)) {
 		err = PTR_ERR(inode);
 		p9_debug(P9_DEBUG_VFS, "inode creation failed %d\n", err);
@@ -340,7 +352,7 @@ static int v9fs_vfs_mkdir_dotl(struct mnt_idmap *idmap,
 	}
 
 	/* instantiate inode and assign the unopened fid to the dentry */
-	inode = v9fs_fid_iget_dotl(dir->i_sb, fid);
+	inode = v9fs_fid_iget_dotl(dir->i_sb, fid, true);
 	if (IS_ERR(inode)) {
 		err = PTR_ERR(inode);
 		p9_debug(P9_DEBUG_VFS, "inode creation failed %d\n",
@@ -776,7 +788,7 @@ v9fs_vfs_mknod_dotl(struct mnt_idmap *idmap, struct inode *dir,
 			 err);
 		goto error;
 	}
-	inode = v9fs_fid_iget_dotl(dir->i_sb, fid);
+	inode = v9fs_fid_iget_dotl(dir->i_sb, fid, true);
 	if (IS_ERR(inode)) {
 		err = PTR_ERR(inode);
 		p9_debug(P9_DEBUG_VFS, "inode creation failed %d\n",

fs/9p/vfs_super.c

@@ -139,7 +139,7 @@ static struct dentry *v9fs_mount(struct file_system_type *fs_type, int flags,
 	else
 		sb->s_d_op = &v9fs_dentry_operations;
 
-	inode = v9fs_get_inode_from_fid(v9ses, fid, sb);
+	inode = v9fs_get_inode_from_fid(v9ses, fid, sb, true);
 	if (IS_ERR(inode)) {
 		retval = PTR_ERR(inode);
 		goto release_sb;