[v2,2/5] shmem: Add dir_operations specific to tmpfs

Commit Message

Chuck Lever May 5, 2023, 6:38 p.m. UTC

From: Chuck Lever <[e-mail]>

Copy the simple directory operations out of fs/libfs.c to create a
set of dir_operations specific to tmpfs. These will be modified by
a subsequent patch to replace the cursor-based directory entry
offset mechanism.

No behavior change is expected.

Signed-off-by: Chuck Lever <[e-mail]>
---
 mm/shmem.c |  143 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 142 insertions(+), 1 deletion(-)

Patch

diff --git a/mm/shmem.c b/mm/shmem.c
index 721f9fd064aa..e48a0947bcaf 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -235,6 +235,7 @@  static const struct super_operations shmem_ops;
 const struct address_space_operations shmem_aops;
 static const struct file_operations shmem_file_operations;
 static const struct inode_operations shmem_inode_operations;
+static const struct file_operations shmem_dir_operations;
 static const struct inode_operations shmem_dir_inode_operations;
 static const struct inode_operations shmem_special_inode_operations;
 static const struct vm_operations_struct shmem_vm_ops;
@@ -2410,7 +2411,7 @@  static struct inode *shmem_get_inode(struct mnt_idmap *idmap, struct super_block
 			/* Some things misbehave if size == 0 on a directory */
 			inode->i_size = 2 * BOGO_DIRENT_SIZE;
 			inode->i_op = &shmem_dir_inode_operations;
-			inode->i_fop = &simple_dir_operations;
+			inode->i_fop = &shmem_dir_operations;
 			break;
 		case S_IFLNK:
 			/*
@@ -3235,6 +3236,137 @@  static const char *shmem_get_link(struct dentry *dentry,
 	return folio_address(folio);
 }
 
+static struct dentry *scan_positives(struct dentry *cursor,
+					struct list_head *p,
+					loff_t count,
+					struct dentry *last)
+{
+	struct dentry *dentry = cursor->d_parent, *found = NULL;
+
+	spin_lock(&dentry->d_lock);
+	while ((p = p->next) != &dentry->d_subdirs) {
+		struct dentry *d = list_entry(p, struct dentry, d_child);
+		// we must at least skip cursors, to avoid livelocks
+		if (d->d_flags & DCACHE_DENTRY_CURSOR)
+			continue;
+		if (simple_positive(d) && !--count) {
+			spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
+			if (simple_positive(d))
+				found = dget_dlock(d);
+			spin_unlock(&d->d_lock);
+			if (likely(found))
+				break;
+			count = 1;
+		}
+		if (need_resched()) {
+			list_move(&cursor->d_child, p);
+			p = &cursor->d_child;
+			spin_unlock(&dentry->d_lock);
+			cond_resched();
+			spin_lock(&dentry->d_lock);
+		}
+	}
+	spin_unlock(&dentry->d_lock);
+	dput(last);
+	return found;
+}
+
+static loff_t shmem_dir_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct dentry *dentry = file->f_path.dentry;
+	switch (whence) {
+		case 1:
+			offset += file->f_pos;
+			fallthrough;
+		case 0:
+			if (offset >= 0)
+				break;
+			fallthrough;
+		default:
+			return -EINVAL;
+	}
+	if (offset != file->f_pos) {
+		struct dentry *cursor = file->private_data;
+		struct dentry *to = NULL;
+
+		inode_lock_shared(dentry->d_inode);
+
+		if (offset > 2)
+			to = scan_positives(cursor, &dentry->d_subdirs,
+					    offset - 2, NULL);
+		spin_lock(&dentry->d_lock);
+		if (to)
+			list_move(&cursor->d_child, &to->d_child);
+		else
+			list_del_init(&cursor->d_child);
+		spin_unlock(&dentry->d_lock);
+		dput(to);
+
+		file->f_pos = offset;
+
+		inode_unlock_shared(dentry->d_inode);
+	}
+	return offset;
+}
+
+/**
+ * shmem_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 shmem_readdir(struct file *file, struct dir_context *ctx)
+{
+	struct dentry *dentry = file->f_path.dentry;
+	struct dentry *cursor = file->private_data;
+	struct list_head *anchor = &dentry->d_subdirs;
+	struct dentry *next = NULL;
+	struct list_head *p;
+
+	if (!dir_emit_dots(file, ctx))
+		return 0;
+
+	if (ctx->pos == 2)
+		p = anchor;
+	else if (!list_empty(&cursor->d_child))
+		p = &cursor->d_child;
+	else
+		return 0;
+
+	while ((next = scan_positives(cursor, p, 1, next)) != NULL) {
+		if (!dir_emit(ctx, next->d_name.name, next->d_name.len,
+			      d_inode(next)->i_ino,
+			      fs_umode_to_dtype(d_inode(next)->i_mode)))
+			break;
+		ctx->pos++;
+		p = &next->d_child;
+	}
+	spin_lock(&dentry->d_lock);
+	if (next)
+		list_move_tail(&cursor->d_child, &next->d_child);
+	else
+		list_del_init(&cursor->d_child);
+	spin_unlock(&dentry->d_lock);
+	dput(next);
+
+	return 0;
+}
+
 #ifdef CONFIG_TMPFS_XATTR
 
 static int shmem_fileattr_get(struct dentry *dentry, struct fileattr *fa)
@@ -3987,6 +4119,15 @@  static const struct inode_operations shmem_inode_operations = {
 #endif
 };
 
+static const struct file_operations shmem_dir_operations = {
+#ifdef CONFIG_TMPFS
+	.llseek		= shmem_dir_llseek,
+	.iterate_shared	= shmem_readdir,
+#endif
+	.read		= generic_read_dir,
+	.fsync		= noop_fsync,
+};
+
 static const struct inode_operations shmem_dir_inode_operations = {
 #ifdef CONFIG_TMPFS
 	.getattr	= shmem_getattr,