Re: [PATCH v3 1/5] ovl: do not open non-data lower file for fsync

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Oct 07, 2024 at 04:19:21PM +0200, Amir Goldstein wrote:
> +static int ovl_real_fdget_path(const struct file *file, struct fd *real,
> +			       struct path *realpath)

> -	if (allow_meta) {
> -		ovl_path_real(dentry, &realpath);
> -	} else {
> -		/* lazy lookup and verify of lowerdata */
> -		err = ovl_verify_lowerdata(dentry);
This check went
> -		if (err)
> -			return err;
> -
> -		ovl_path_realdata(dentry, &realpath);
> -	}

> @@ -138,7 +129,33 @@ static int ovl_real_fdget(const struct file *file, struct fd *real)
>  		return 0;
>  	}
>  
> -	return ovl_real_fdget_meta(file, real, false);
> +	/* lazy lookup and verify of lowerdata */
> +	err = ovl_verify_lowerdata(dentry);

... here

> +	if (err)
> +		return err;
> +
> +	ovl_path_realdata(dentry, &realpath);

> +static int ovl_upper_fdget(const struct file *file, struct fd *real, bool data)
> +{
> +	struct dentry *dentry = file_dentry(file);
> +	struct path realpath;
> +	enum ovl_path_type type;
> +
> +	if (data)
> +		type = ovl_path_realdata(dentry, &realpath);

... but not here.

I can see the point of not doing that in ->fsync() after we'd already
done ovl_verify_lowerdata() at open time, but what's different about
->read_iter() and friends that also come only after ->open()?
IOW, why is fdatasync() different from other data-access cases?