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 14:11:41 +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 Wed, Jan 23, 2013 at 12:14:32AM +0100, Jan Kara wrote:
> On Wed 23-01-13 00:00:17, Zheng Liu wrote:
> > 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.
>   No, the patch is wrong. The code before the patch is correct. We can get
> to that code for extent based inode which has unwritten conversions pending
> and we need to wait for those as otherwise we could return 0s in places
> where we acknowledged successful write just a while ago. Or am I missing
> something?

Ah, I see.  I guess that the problem is that the dio read races with buffered
write.

   dio read                         buffered write
                                    ->ext4_file_write
                                      ->ext4_da_write_begin
                                      ->ext4_da_write_end
                                      [buffered write has finished, but the data
                                       and metadata has not been flushed]
   ->generic_file_aio_read
     ->filemap_write_and_wait_range
       ->do_writepages
         ->ext4_da_writepages
     ->filemap_fdatawait_range
       ->wait_on_page_writeback
                                    ->ext4_end_bio
                                      ->end_page_writeback
                                        [unwritten extent has not been
                                         converted]
     ->ext4_ind_direct_IO
       [here we need to flush unwritten io]

So it seems that this patch could be applied after reworking unwritten extent
conversion.

FWIW, after applied this patch, the latency of dio read could be reduced
dramatically.  So that would be great if this patch can be applied when it
doesn't break something.

Thanks,
						- Zheng
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