Re: [PATCH v3 1/3] btrfs: kill __btrfs_set_prop()

[David Sterba <dsterba@suse.cz>]

On Tue, Feb 12, 2019 at 10:18:49AM +0800, Anand Jain wrote:
> btrfs_set_prop() is a redirect to __btrfs_set_prop() with the
> transaction handler equal to NULL. And __btrfs_set_prop() inturn diectly
> uses trans to do_setxattr() which when trans is NULL creates a transaction.

That's right and I think that some of the callsites could actually pass
the existing transaction to btrfs_set_prop instead of relying on the
fallback behaviour.

Also, the potential NULL as transaction handle is not a good thing from
the API point of view and I'd like to reduce that to minimum (there are
some justified cases).

> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 3f9d7be30bf4..5a4ed2f66e09 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -284,7 +284,8 @@ static int btrfs_ioctl_setflags(struct file *file, void __user *arg)
>  		binode->flags &= ~BTRFS_INODE_COMPRESS;
>  		binode->flags |= BTRFS_INODE_NOCOMPRESS;
>  
> -		ret = btrfs_set_prop(inode, "btrfs.compression", NULL, 0, 0);
> +		ret = btrfs_set_prop(NULL, inode, "btrfs.compression", NULL, 0,
> +				     0);
>  		if (ret && ret != -ENODATA)
>  			goto out_drop;
>  	} else if (fsflags & FS_COMPR_FL) {
> @@ -302,13 +303,14 @@ static int btrfs_ioctl_setflags(struct file *file, void __user *arg)
>  		if (!comp || comp[0] == 0)
>  			comp = btrfs_compress_type2str(BTRFS_COMPRESS_ZLIB);
>  
> -		ret = btrfs_set_prop(inode, "btrfs.compression",
> -				     comp, strlen(comp), 0);
> +		ret = btrfs_set_prop(NULL, inode, "btrfs.compression", comp,
> +				     strlen(comp), 0);
>  		if (ret)
>  			goto out_drop;
>  
>  	} else {
> -		ret = btrfs_set_prop(inode, "btrfs.compression", NULL, 0, 0);
> +		ret = btrfs_set_prop(NULL, inode, "btrfs.compression", NULL, 0,
> +				     0);
>  		if (ret && ret != -ENODATA)
>  			goto out_drop;
>  		binode->flags &= ~(BTRFS_INODE_COMPRESS | BTRFS_INODE_NOCOMPRESS);

When the if-else block ends, there's a new transaction started, this
seems unnecessary.

> diff --git a/fs/btrfs/props.c b/fs/btrfs/props.c
> index dc6140013ae8..4525a2a4d1cd 100644
> --- a/fs/btrfs/props.c
> +++ b/fs/btrfs/props.c
> @@ -85,12 +85,9 @@ static const struct hlist_head *find_prop_handlers_by_hash(const u64 hash)
>  	return NULL;
>  }
>  
> -static int __btrfs_set_prop(struct btrfs_trans_handle *trans,
> -			    struct inode *inode,
> -			    const char *name,
> -			    const char *value,
> -			    size_t value_len,
> -			    int flags)
> +int btrfs_set_prop(struct btrfs_trans_handle *trans, struct inode *inode,
> +		   const char *name, const char *value, size_t value_len,
> +		   int flags)
>  {
>  	const struct prop_handler *handler;
>  	int ret;
> @@ -133,15 +130,6 @@ static int __btrfs_set_prop(struct btrfs_trans_handle *trans,
>  	return 0;
>  }
>  
> -int btrfs_set_prop(struct inode *inode,
> -		   const char *name,
> -		   const char *value,
> -		   size_t value_len,
> -		   int flags)
> -{
> -	return __btrfs_set_prop(NULL, inode, name, value, value_len, flags);

I agree that one function would be better here, with defined semantics
of 'trans'.

There are more cleanups around the properties, also in the xattr
handling functions. This patchset is a step in the right direction and I
think the cleanups could be more extensive.

The idea for the xattr function:

- the VFS callbacks (like btrfs_xattr_handler_set_prop) will start the
  transaction themselves and pass the handle to the prop function

- a xattr function that does not take a valid trans could be named like
  btrfs_setxattr_notrans and will start the transaction, ie. lifting
  that to the callers

- there's a maze of the xattr callbacks so assertions are needed