linux-kernel - Re: mnt_want_write

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Message-Id: <1249413319.11056.131.camel@nimitz>
Date:	Tue, 04 Aug 2009 12:15:19 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Al Viro <viro@...iv.linux.org.uk>, Nick Piggin <npiggin@...e.de>,
	linux-kernel@...r.kernel.org, akpm <akpm@...uxfoundation.org>
Subject: Re: mnt_want_write_file() has problem?

On Mon, 2009-08-03 at 06:36 +0900, OGAWA Hirofumi wrote:
> diff -puN fs/namespace.c~mnt_want_write-wrong-assume fs/namespace.c
> ---
> linux-2.6/fs/namespace.c~mnt_want_write-wrong-assume        2009-08-03
> 04:33:35.000000000 +0900
> +++ linux-2.6-hirofumi/fs/namespace.c   2009-08-03 04:31:34.000000000
> +0900
> @@ -316,7 +316,8 @@ EXPORT_SYMBOL_GPL(mnt_clone_write);
>   */
>  int mnt_want_write_file(struct file *file)
>  {
> -       if (!(file->f_mode & FMODE_WRITE))
> +       struct inode *inode = file->f_dentry->d_inode;
> +       if (!(file->f_mode & FMODE_WRITE) || special_file(inode->i_mode))
>                 return mnt_want_write(file->f_path.mnt);
>         else
>                 return mnt_clone_write(file->f_path.mnt);

I'm fine with this.  I'd like a debugging check in mnt_clone_write()
since this bug is easy to detect, but such a check will also cost all of
the performance gains that Nick added.  So, we can't have it
unconditionally.

-- 

Signed-off-by: Dave Hansen <dave@...ux.vnet.ibm.com>

-- Dave

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