Re: [PATCH 3/3] overlayfs: Report writeback errors on upper

[Matthew Wilcox]

On Mon, Dec 21, 2020 at 02:50:55PM -0500, Vivek Goyal wrote:
> +static int ovl_errseq_check_advance(struct super_block *sb, struct file *file)
> +{
> +	struct ovl_fs *ofs = sb->s_fs_info;
> +	struct super_block *upper_sb;
> +	int ret;
> +
> +	if (!ovl_upper_mnt(ofs))
> +		return 0;
> +
> +	upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
> +
> +	if (!errseq_check(&upper_sb->s_wb_err, file->f_sb_err))
> +		return 0;
> +
> +	/* Something changed, must use slow path */
> +	spin_lock(&file->f_lock);
> +	ret = errseq_check_and_advance(&upper_sb->s_wb_err, &file->f_sb_err);
> +	spin_unlock(&file->f_lock);

Why are you microoptimising syncfs()?  Are there really applications which
call syncfs() in a massively parallel manner on the same file descriptor?