libfs: Add directory operations for stable offsets

Create a vector of directory operations in fs/libfs.c that handles
directory seeks and readdir via stable offsets instead of the
current cursor-based mechanism.

For the moment these are unused.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Message-Id: <168814732984.530310.11190772066786107220.stgit@manet.1015granger.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/filesystems/locking.rst

@@ -85,13 +85,14 @@ prototypes::
 			    struct dentry *dentry, struct fileattr *fa);
 	int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
 	struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);
+	struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
 
 locking rules:
 	all may block
 
-==============	=============================================
+==============	==================================================
 ops		i_rwsem(inode)
-==============	=============================================
+==============	==================================================
 lookup:		shared
 create:		exclusive
 link:		exclusive (both)
@@ -115,7 +116,8 @@ atomic_open:	shared (exclusive if O_CREAT is set in open flags)
 tmpfile:	no
 fileattr_get:	no or exclusive
 fileattr_set:	exclusive
-==============	=============================================
+get_offset_ctx  no
+==============	==================================================
 
 
 	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem

Documentation/filesystems/vfs.rst

@@ -515,6 +515,7 @@ As of kernel 2.6.22, the following members are defined:
 		int (*fileattr_set)(struct mnt_idmap *idmap,
 				    struct dentry *dentry, struct fileattr *fa);
 		int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
+	        struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
 	};
 
 Again, all methods are called without any locks being held, unless
@@ -675,7 +676,10 @@ otherwise noted.
 	called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
 	change miscellaneous file flags and attributes.  Callers hold
 	i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().
-
+``get_offset_ctx``
+	called to get the offset context for a directory inode. A
+        filesystem must define this operation to use
+        simple_offset_dir_operations.
 
 The Address Space Object
 ========================

fs/libfs.c

@@ -239,6 +239,254 @@ const struct inode_operations simple_dir_inode_operations = {
 };
 EXPORT_SYMBOL(simple_dir_inode_operations);
 
+static void offset_set(struct dentry *dentry, u32 offset)
+{
+	dentry->d_fsdata = (void *)((uintptr_t)(offset));
+}
+
+static u32 dentry2offset(struct dentry *dentry)
+{
+	return (u32)((uintptr_t)(dentry->d_fsdata));
+}
+
+/**
+ * simple_offset_init - initialize an offset_ctx
+ * @octx: directory offset map to be initialized
+ *
+ */
+void simple_offset_init(struct offset_ctx *octx)
+{
+	xa_init_flags(&octx->xa, XA_FLAGS_ALLOC1);
+
+	/* 0 is '.', 1 is '..', so always start with offset 2 */
+	octx->next_offset = 2;
+}
+
+/**
+ * simple_offset_add - Add an entry to a directory's offset map
+ * @octx: directory offset ctx to be updated
+ * @dentry: new dentry being added
+ *
+ * Returns zero on success. @so_ctx and the dentry offset are updated.
+ * Otherwise, a negative errno value is returned.
+ */
+int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
+{
+	static const struct xa_limit limit = XA_LIMIT(2, U32_MAX);
+	u32 offset;
+	int ret;
+
+	if (dentry2offset(dentry) != 0)
+		return -EBUSY;
+
+	ret = xa_alloc_cyclic(&octx->xa, &offset, dentry, limit,
+			      &octx->next_offset, GFP_KERNEL);
+	if (ret < 0)
+		return ret;
+
+	offset_set(dentry, offset);
+	return 0;
+}
+
+/**
+ * simple_offset_remove - Remove an entry to a directory's offset map
+ * @octx: directory offset ctx to be updated
+ * @dentry: dentry being removed
+ *
+ */
+void simple_offset_remove(struct offset_ctx *octx, struct dentry *dentry)
+{
+	u32 offset;
+
+	offset = dentry2offset(dentry);
+	if (offset == 0)
+		return;
+
+	xa_erase(&octx->xa, offset);
+	offset_set(dentry, 0);
+}
+
+/**
+ * simple_offset_rename_exchange - exchange rename with directory offsets
+ * @old_dir: parent of dentry being moved
+ * @old_dentry: dentry being moved
+ * @new_dir: destination parent
+ * @new_dentry: destination dentry
+ *
+ * Returns zero on success. Otherwise a negative errno is returned and the
+ * rename is rolled back.
+ */
+int simple_offset_rename_exchange(struct inode *old_dir,
+				  struct dentry *old_dentry,
+				  struct inode *new_dir,
+				  struct dentry *new_dentry)
+{
+	struct offset_ctx *old_ctx = old_dir->i_op->get_offset_ctx(old_dir);
+	struct offset_ctx *new_ctx = new_dir->i_op->get_offset_ctx(new_dir);
+	u32 old_index = dentry2offset(old_dentry);
+	u32 new_index = dentry2offset(new_dentry);
+	int ret;
+
+	simple_offset_remove(old_ctx, old_dentry);
+	simple_offset_remove(new_ctx, new_dentry);
+
+	ret = simple_offset_add(new_ctx, old_dentry);
+	if (ret)
+		goto out_restore;
+
+	ret = simple_offset_add(old_ctx, new_dentry);
+	if (ret) {
+		simple_offset_remove(new_ctx, old_dentry);
+		goto out_restore;
+	}
+
+	ret = simple_rename_exchange(old_dir, old_dentry, new_dir, new_dentry);
+	if (ret) {
+		simple_offset_remove(new_ctx, old_dentry);
+		simple_offset_remove(old_ctx, new_dentry);
+		goto out_restore;
+	}
+	return 0;
+
+out_restore:
+	offset_set(old_dentry, old_index);
+	xa_store(&old_ctx->xa, old_index, old_dentry, GFP_KERNEL);
+	offset_set(new_dentry, new_index);
+	xa_store(&new_ctx->xa, new_index, new_dentry, GFP_KERNEL);
+	return ret;
+}
+
+/**
+ * simple_offset_destroy - Release offset map
+ * @octx: directory offset ctx that is about to be destroyed
+ *
+ * During fs teardown (eg. umount), a directory's offset map might still
+ * contain entries. xa_destroy() cleans out anything that remains.
+ */
+void simple_offset_destroy(struct offset_ctx *octx)
+{
+	xa_destroy(&octx->xa);
+}
+
+/**
+ * offset_dir_llseek - Advance the read position of a directory descriptor
+ * @file: an open directory whose position is to be updated
+ * @offset: a byte offset
+ * @whence: enumerator describing the starting position for this update
+ *
+ * SEEK_END, SEEK_DATA, and SEEK_HOLE are not supported for directories.
+ *
+ * Returns the updated read position if successful; otherwise a
+ * negative errno is returned and the read position remains unchanged.
+ */
+static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
+{
+	switch (whence) {
+	case SEEK_CUR:
+		offset += file->f_pos;
+		fallthrough;
+	case SEEK_SET:
+		if (offset >= 0)
+			break;
+		fallthrough;
+	default:
+		return -EINVAL;
+	}
+
+	return vfs_setpos(file, offset, U32_MAX);
+}
+
+static struct dentry *offset_find_next(struct xa_state *xas)
+{
+	struct dentry *child, *found = NULL;
+
+	rcu_read_lock();
+	child = xas_next_entry(xas, U32_MAX);
+	if (!child)
+		goto out;
+	spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
+	if (simple_positive(child))
+		found = dget_dlock(child);
+	spin_unlock(&child->d_lock);
+out:
+	rcu_read_unlock();
+	return found;
+}
+
+static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
+{
+	u32 offset = dentry2offset(dentry);
+	struct inode *inode = d_inode(dentry);
+
+	return ctx->actor(ctx, dentry->d_name.name, dentry->d_name.len, offset,
+			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
+}
+
+static void offset_iterate_dir(struct dentry *dir, struct dir_context *ctx)
+{
+	struct inode *inode = d_inode(dir);
+	struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
+	XA_STATE(xas, &so_ctx->xa, ctx->pos);
+	struct dentry *dentry;
+
+	while (true) {
+		spin_lock(&dir->d_lock);
+		dentry = offset_find_next(&xas);
+		spin_unlock(&dir->d_lock);
+		if (!dentry)
+			break;
+
+		if (!offset_dir_emit(ctx, dentry)) {
+			dput(dentry);
+			break;
+		}
+
+		dput(dentry);
+		ctx->pos = xas.xa_index + 1;
+	}
+}
+
+/**
+ * offset_readdir - Emit entries starting at offset @ctx->pos
+ * @file: an open directory to iterate over
+ * @ctx: directory iteration context
+ *
+ * Caller must hold @file's i_rwsem to prevent insertion or removal of
+ * entries during this call.
+ *
+ * On entry, @ctx->pos contains an offset that represents the first entry
+ * to be read from the directory.
+ *
+ * The operation continues until there are no more entries to read, or
+ * until the ctx->actor indicates there is no more space in the caller's
+ * output buffer.
+ *
+ * On return, @ctx->pos contains an offset that will read the next entry
+ * in this directory when shmem_readdir() is called again with @ctx.
+ *
+ * Return values:
+ *   %0 - Complete
+ */
+static int offset_readdir(struct file *file, struct dir_context *ctx)
+{
+	struct dentry *dir = file->f_path.dentry;
+
+	lockdep_assert_held(&d_inode(dir)->i_rwsem);
+
+	if (!dir_emit_dots(file, ctx))
+		return 0;
+
+	offset_iterate_dir(dir, ctx);
+	return 0;
+}
+
+const struct file_operations simple_offset_dir_operations = {
+	.llseek		= offset_dir_llseek,
+	.iterate_shared	= offset_readdir,
+	.read		= generic_read_dir,
+	.fsync		= noop_fsync,
+};
+
 static struct dentry *find_next_child(struct dentry *parent, struct dentry *prev)
 {
 	struct dentry *child = NULL;

include/linux/fs.h

@@ -1770,6 +1770,7 @@ struct dir_context {
 
 struct iov_iter;
 struct io_uring_cmd;
+struct offset_ctx;
 
 struct file_operations {
 	struct module *owner;
@@ -1857,6 +1858,7 @@ struct inode_operations {
 	int (*fileattr_set)(struct mnt_idmap *idmap,
 			    struct dentry *dentry, struct fileattr *fa);
 	int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
+	struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
 } ____cacheline_aligned;
 
 static inline ssize_t call_read_iter(struct file *file, struct kiocb *kio,
@@ -2971,6 +2973,22 @@ extern ssize_t simple_read_from_buffer(void __user *to, size_t count,
 extern ssize_t simple_write_to_buffer(void *to, size_t available, loff_t *ppos,
 		const void __user *from, size_t count);
 
+struct offset_ctx {
+	struct xarray		xa;
+	u32			next_offset;
+};
+
+void simple_offset_init(struct offset_ctx *octx);
+int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry);
+void simple_offset_remove(struct offset_ctx *octx, struct dentry *dentry);
+int simple_offset_rename_exchange(struct inode *old_dir,
+				  struct dentry *old_dentry,
+				  struct inode *new_dir,
+				  struct dentry *new_dentry);
+void simple_offset_destroy(struct offset_ctx *octx);
+
+extern const struct file_operations simple_offset_dir_operations;
+
 extern int __generic_file_fsync(struct file *, loff_t, loff_t, int);
 extern int generic_file_fsync(struct file *, loff_t, loff_t, int);