Re: [PATCH v3 1/3] libfs: Add directory operations for stable offsets

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 6/24/23 00:21, Bernd Schubert wrote:
> 
> 
> On 6/6/23 15:10, Chuck Lever wrote:
>> From: Chuck Lever <chuck.lever@oracle.com>
>>
>> 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>
>> ---
>>   fs/dcache.c            |    1
>>   fs/libfs.c             |  185 
>> ++++++++++++++++++++++++++++++++++++++++++++++++
>>   include/linux/dcache.h |    1
>>   include/linux/fs.h     |    9 ++
>>   4 files changed, 196 insertions(+)
>>
>> diff --git a/fs/dcache.c b/fs/dcache.c
>> index 52e6d5fdab6b..9c9a801f3b33 100644
>> --- a/fs/dcache.c
>> +++ b/fs/dcache.c
>> @@ -1813,6 +1813,7 @@ static struct dentry *__d_alloc(struct 
>> super_block *sb, const struct qstr *name)
>>       dentry->d_sb = sb;
>>       dentry->d_op = NULL;
>>       dentry->d_fsdata = NULL;
>> +    dentry->d_offset = 0;
>>       INIT_HLIST_BL_NODE(&dentry->d_hash);
>>       INIT_LIST_HEAD(&dentry->d_lru);
>>       INIT_LIST_HEAD(&dentry->d_subdirs);
>> diff --git a/fs/libfs.c b/fs/libfs.c
>> index 89cf614a3271..07317bbe1668 100644
>> --- a/fs/libfs.c
>> +++ b/fs/libfs.c
>> @@ -239,6 +239,191 @@ const struct inode_operations 
>> simple_dir_inode_operations = {
>>   };
>>   EXPORT_SYMBOL(simple_dir_inode_operations);
>> +/**
>> + * stable_offset_init - initialize a parent directory
>> + * @dir: parent directory to be initialized
>> + *
>> + */
>> +void stable_offset_init(struct inode *dir)
>> +{
>> +    xa_init_flags(&dir->i_doff_map, XA_FLAGS_ALLOC1);
>> +    dir->i_next_offset = 0;
>> +}
>> +EXPORT_SYMBOL(stable_offset_init);
>> +
>> +/**
>> + * stable_offset_add - Add an entry to a directory's stable offset map
>> + * @dir: parent directory being modified
>> + * @dentry: new dentry being added
>> + *
>> + * Returns zero on success. Otherwise, a negative errno value is 
>> returned.
>> + */
>> +int stable_offset_add(struct inode *dir, struct dentry *dentry)
>> +{
>> +    struct xa_limit limit = XA_LIMIT(2, U32_MAX);
>> +    u32 offset = 0;
>> +    int ret;
>> +
>> +    if (dentry->d_offset)
>> +        return -EBUSY;
>> +
>> +    ret = xa_alloc_cyclic(&dir->i_doff_map, &offset, dentry, limit,
>> +                  &dir->i_next_offset, GFP_KERNEL);
> 
> Please see below at struct inode my question about i_next_offset.
> 
>> +    if (ret < 0)
>> +        return ret;
>> +
>> +    dentry->d_offset = offset;
>> +    return 0;
>> +}
>> +EXPORT_SYMBOL(stable_offset_add);
>> +
>> +/**
>> + * stable_offset_remove - Remove an entry to a directory's stable 
>> offset map
>> + * @dir: parent directory being modified
>> + * @dentry: dentry being removed
>> + *
>> + */
>> +void stable_offset_remove(struct inode *dir, struct dentry *dentry)
>> +{
>> +    if (!dentry->d_offset)
>> +        return;
>> +
>> +    xa_erase(&dir->i_doff_map, dentry->d_offset);
>> +    dentry->d_offset = 0;
>> +}
>> +EXPORT_SYMBOL(stable_offset_remove);
>> +
>> +/**
>> + * stable_offset_destroy - Release offset map
>> + * @dir: parent directory 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 stable_offset_destroy(struct inode *dir)
>> +{
>> +    xa_destroy(&dir->i_doff_map);
>> +}
>> +EXPORT_SYMBOL(stable_offset_destroy);
>> +
>> +/**
>> + * stable_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 stable_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 *stable_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;
>> +}

I wonder if this should try the next dentry when simple_positive() 
returns false, what is if there is a readdir/unlink race? readdir now 
abort in the middle instead of continuing with the next dentry?

>> +
>> +static bool stable_dir_emit(struct dir_context *ctx, struct dentry 
>> *dentry)
>> +{
>> +    struct inode *inode = d_inode(dentry);
>> +
>> +    return ctx->actor(ctx, dentry->d_name.name, dentry->d_name.len,
>> +              dentry->d_offset, inode->i_ino,
>> +              fs_umode_to_dtype(inode->i_mode));
>> +}
>> +
>> +static void stable_iterate_dir(struct dentry *dir, struct dir_context 
>> *ctx)
>> +{
>> +    XA_STATE(xas, &((d_inode(dir))->i_doff_map), ctx->pos);
>> +    struct dentry *dentry;
>> +
>> +    while (true) {
>> +        spin_lock(&dir->d_lock);
>> +        dentry = stable_find_next(&xas);
>> +        spin_unlock(&dir->d_lock);
>> +        if (!dentry)
>> +            break;
>> +
>> +        if (!stable_dir_emit(ctx, dentry)) {
>> +            dput(dentry);
>> +            break;
>> +        }
>> +
>> +        dput(dentry);
>> +        ctx->pos = xas.xa_index + 1;
>> +    }
>> +}
>> +
>> +/**
>> + * stable_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 stable_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;
>> +
>> +    stable_iterate_dir(dir, ctx);
>> +    return 0;
>> +}
>> +
>> +const struct file_operations stable_dir_operations = {
>> +    .llseek        = stable_dir_llseek,
>> +    .iterate_shared    = stable_readdir,
>> +    .read        = generic_read_dir,
>> +    .fsync        = noop_fsync,
>> +};
>> +EXPORT_SYMBOL(stable_dir_operations);
>> +
>>   static struct dentry *find_next_child(struct dentry *parent, struct 
>> dentry *prev)
>>   {
>>       struct dentry *child = NULL;
>> diff --git a/include/linux/dcache.h b/include/linux/dcache.h
>> index 6b351e009f59..579ce1800efe 100644
>> --- a/include/linux/dcache.h
>> +++ b/include/linux/dcache.h
>> @@ -96,6 +96,7 @@ struct dentry {
>>       struct super_block *d_sb;    /* The root of the dentry tree */
>>       unsigned long d_time;        /* used by d_revalidate */
>>       void *d_fsdata;            /* fs-specific data */
>> +    u32 d_offset;            /* directory offset in parent */
>>       union {
>>           struct list_head d_lru;        /* LRU list */
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 133f0640fb24..3fc2c04ed8ff 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -719,6 +719,10 @@ struct inode {
>>   #endif
>>       void            *i_private; /* fs or device private pointer */
>> +
>> +    /* simplefs stable directory offset tracking */
>> +    struct xarray        i_doff_map;
>> +    u32            i_next_offset;
> 
> Hmm, I was grepping through the patches and only find that 
> "i_next_offset" is initialized to 0 and then passed to xa_alloc_cyclic - 
> does this really need to part of struct inode or could it be a local 
> variable in stable_offset_add()?

Hmm, at best that is used for rename, but then is this offset right when 
old_dir and new_dir differ?

> 
> I only managed to look a bit through the patches right now, personally I 
> like v2 better as it doesn't extend struct inode with changes that can 
> be used by in-memory file system only. What do others think? An 
> alternative would be to have these fields in struct shmem_inode_info and 
> pass it as extra argument to the stable_ functions?
> 
> 
> Thanks,
> Bernd