On 6/28/23 17:25, Chuck Lever wrote:
From: Chuck Lever <chuck.lever@xxxxxxxxxx>
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@xxxxxxxxxx>
---
fs/libfs.c | 247 ++++++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/fs.h | 18 ++++
2 files changed, 265 insertions(+)
diff --git a/fs/libfs.c b/fs/libfs.c
index 89cf614a3271..2b0d5ac472df 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -239,6 +239,253 @@ const struct inode_operations simple_dir_inode_operations = {
};
EXPORT_SYMBOL(simple_dir_inode_operations);
+static void offset_set(struct dentry *dentry, unsigned long offset)
+{
+ dentry->d_fsdata = (void *)offset;
+}
+
+static unsigned long dentry2offset(struct dentry *dentry)
+{
+ return (unsigned long)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)
+{
+ unsigned long index = dentry2offset(dentry);
+
+ if (index == 0)
+ return;
+
+ xa_erase(&octx->xa, index);
+ 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);
+ unsigned long old_index = dentry2offset(old_dentry);
+ unsigned long 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;
+}
Thanks for the update, looks great!
+
+/**
+ * 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)
+{
+ loff_t 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;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 133f0640fb24..85de389e4eb8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1767,6 +1767,7 @@ struct dir_context {
struct iov_iter;
struct io_uring_cmd;
+struct offset_ctx;
struct file_operations {
struct module *owner;
@@ -1854,6 +1855,7 @@ 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;
Should this be documented in filesystems/vfs.rst and
filesystems/locking.rst?
Thanks,
Bernd
