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

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Message-ID: <20130122152243.GB32366@quack.suse.cz>
Date:	Tue, 22 Jan 2013 16:22:43 +0100
From:	Jan Kara <jack@...e.cz>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	Jan Kara <jack@...e.cz>, 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 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.

							Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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