linux-ext4 - Re: [PATCH 03/12] ext4: Remove bogus wait for unwritten extents in ext4_ind_direct

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Date:	Wed, 23 Jan 2013 00:00:17 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Dmitry Monakhov <dmonakhov@...nvz.org>, Ted Tso <tytso@....edu>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 03/12] ext4: Remove bogus wait for unwritten extents in
 ext4_ind_direct_IO

On Tue, Jan 22, 2013 at 04:22:43PM +0100, Jan Kara wrote:
> On Tue 22-01-13 22:22:21, Zheng Liu wrote:
> > On Tue, Jan 22, 2013 at 02:44:00PM +0100, Jan Kara wrote:
> > > On Tue 22-01-13 15:11:24, Dmitry Monakhov wrote:
> > > > On Fri, 18 Jan 2013 13:00:37 +0100, Jan Kara <jack@...e.cz> wrote:
> > > > > When using indirect blocks there is no possibility to have any unwritten
> > > > > extents. So wait for them in ext4_ind_direct_IO() is just bogus.
> > > > But as soon as i remember indirect implementation may also be used by
> > > > extents based inodes 3074: ext4_ext_direct_IO
> > > >     /* Use the old path for reads and writes beyond i_size. */
> > > >     if (rw != WRITE || final_size > inode->i_size)
> > > >        return ext4_ind_direct_IO(rw, iocb, iov, offset, nr_segs);
> > > > 
> > > > Am I missing ?
> > >   Ah, that's a catch. Thanks for pointing that out! So my patch is wrong
> > > and that code path needs some cleaning and commenting. In particular I'm
> > > afraid using dioread_nolock for inodes with indirect map causes data
> > > exposure bugs when unlocked DIO read races with DIO write because such
> > > inodes don't support uninitialized extents.
> > 
> > Sorry, but I am still confused.  dioread_nolock is only for extent-based
> > file.  So when a file system without extent feature, dioread_nolock
> > couldn't be enabled.  It seems that we don't need to worry about
> > exposing stale data here.
>   Well, you can have fs with extent feature enabled but still with inodes
> using indirect map. But as Dmitry pointed out, ext4_should_dioread_nolock()
> handles that correctly. So there's not a bug I was suspecting.

Yep, the patch itself is fine.  But that would be great if a comment is
added here.

Regards,
                                                - Zheng
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