linux-kernel - Re: [PATCH 4/9] vfs: intercept reads to overlay files

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Date:   Mon, 20 Feb 2017 15:47:03 +0800
From:   "zhangyi (F)" <yi.zhang@...wei.com>
To:     <mszeredi@...hat.com>
CC:     <linux-unionfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>,
        <miaoxie@...wei.com>
Subject: Re: [PATCH 4/9] vfs: intercept reads to overlay files

On 2017/2/18 00:10, Miklos Szeredi wrote:
> diff --git a/fs/open.c b/fs/open.c
> index 9921f70bc5ca..4916ccff29f5 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -762,6 +762,8 @@ static int do_dentry_open(struct file *f,
>  	if ((f->f_mode & FMODE_WRITE) &&
>  	     likely(f->f_op->write || f->f_op->write_iter))
>  		f->f_mode |= FMODE_CAN_WRITE;
> +	if (unlikely(d_inode(f->f_path.dentry) != inode))
> +		f->f_mode |= FMODE_OVERLAY;

Can we just add flag to the "readonly && not copied" file, not all overlayfs files?
Beacuse we just want to check the ro file before copied-up.

>  	f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
>  
> diff --git a/fs/overlay_util.c b/fs/overlay_util.c new file mode 100644 index 000000000000..0daff19bad0b
> --- /dev/null
> +++ b/fs/overlay_util.c
> @@ -0,0 +1,39 @@
> +/*
> + * Copyright (C) 2017 Red Hat, Inc.
> + *
> + * This program is free software; you can redistribute it and/or modify 
> +it
> + * under the terms of the GNU General Public License version 2 as 
> +published by
> + * the Free Software Foundation.
> + */
> +#if IS_ENABLED(CONFIG_OVERLAY_FS)
> +
> +#include <linux/overlay_util.h>
> +#include <linux/fs.h>
> +#include <linux/file.h>
> +#include "internal.h"
> +
> +static bool overlay_file_consistent(struct file *file) {
> +	return d_real_inode(file->f_path.dentry) == file_inode(file); }
> +
> +ssize_t overlay_read_iter(struct file *file, struct kiocb *kio,
> +			  struct iov_iter *iter)
> +{
> +	ssize_t ret;
> +
> +	if (likely(overlay_file_consistent(file)))
> +		return file->f_op->read_iter(kio, iter);
> +
> +	file = filp_clone_open(file);
> +	if (IS_ERR(file))
> +		return PTR_ERR(file);
> +
> +	ret = vfs_iter_read(file, iter, &kio->ki_pos);
> +	fput(file);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(overlay_read_iter);

Can we try to replace the old file with the new file, and then clear the f_mode flag we added?
If so, we can avoid opening file for each reading call and avoid copied file consistency check.

Thanks.

zhangyi