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

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

On 6/26/23 15:36, Bernd Schubert wrote:
> 
> 
> 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?

Sorry, ignore that please, as I wrote before, I had read it a per dentry 
value.

> 
>>
>> 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