linux-ext4 - Re: [PATCH v3 2/5] fs: Add inode_update_time

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Message-ID: <20130820033329.GG6023@dastard>
Date:	Tue, 20 Aug 2013 13:33:29 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Theodore Ts'o <tytso@....edu>,
	Dave Hansen <dave.hansen@...ux.intel.com>, xfs@....sgi.com,
	Jan Kara <jack@...e.cz>, Tim Chen <tim.c.chen@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v3 2/5] fs: Add inode_update_time_writable

On Mon, Aug 19, 2013 at 08:20:12PM -0700, Andy Lutomirski wrote:
> On Mon, Aug 19, 2013 at 7:28 PM, Dave Chinner <david@...morbit.com> wrote:
> > On Fri, Aug 16, 2013 at 04:22:09PM -0700, Andy Lutomirski wrote:
> >> This is like file_update_time, except that it acts on a struct inode *
> >> instead of a struct file *.
> >>
> >> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> >> ---
> >>  fs/inode.c         | 72 ++++++++++++++++++++++++++++++++++++++++++------------
> >>  include/linux/fs.h |  1 +
> >>  2 files changed, 58 insertions(+), 15 deletions(-)
> >>
> 
> [...]
> 
> >> +
> >> +int inode_update_time_writable(struct inode *inode)
> >> +{
> >> +     struct timespec now;
> >> +     int sync_it = prepare_update_cmtime(inode, &now);
> >> +     int ret;
> >> +
> >> +     if (!sync_it)
> >> +             return 0;
> >> +
> >> +     /* sb_start_pagefault and update_time can both sleep. */
> >> +     sb_start_pagefault(inode->i_sb);
> >> +     ret = update_time(inode, &now, sync_it);
> >> +     sb_end_pagefault(inode->i_sb);
> >
> > This gets called from the writeback path - you can't use
> > sb_start_pagefault/sb_end_pagefault in that path.
> 
> The race I'm worried about is:
> 
>  - mmap
>  - write to the mapping
>  - remount ro
>  - flush_cmtime -> inode_update_time_writable

sb_start_pagefault() is for filesystem freeze protection, not
remount-ro protection. If you freeze the filesystem, then we stop
writes and pagefaults by making sb_start_pagefault/sb_start_write
block, and then run writeback to clean all the pages.  If writeback
then blocks on sb_start_pagefault(), we've got a deadlock.

> This may be impossible, in which case I'm okay, but it's nice to have
> a sanity check.  I'll see if I can figure out how to do that.

The process of remount-ro should flush the dirty pages - the inode
and page has been marked dirty by page_mkwrite(), after all.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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