From: Josef Bacik <jbacik@fusionio.com>
To: Miao Xie <miaox@cn.fujitsu.com>
Cc: Josef Bacik <JBacik@fusionio.com>,
Linux Btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 1/2] Btrfs: serialize unlocked dio reads with truncate
Date: Fri, 1 Feb 2013 09:40:36 -0500 [thread overview]
Message-ID: <20130201144036.GC11835@localhost.localdomain> (raw)
In-Reply-To: <510B5912.7000908@cn.fujitsu.com>
On Thu, Jan 31, 2013 at 10:56:34PM -0700, Miao Xie wrote:
> On Thu, 31 Jan 2013 11:40:41 -0500, Josef Bacik wrote:
> > On Thu, Jan 31, 2013 at 02:23:19AM -0700, Miao Xie wrote:
> >> Currently, we can do unlocked dio reads, but the following race
> >> is possible:
> >>
> >> dio_read_task truncate_task
> >> ->btrfs_setattr()
> >> ->btrfs_direct_IO
> >> ->__blockdev_direct_IO
> >> ->btrfs_get_block
> >> ->btrfs_truncate()
> >> #alloc truncated blocks
> >> #to other inode
> >> ->submit_io()
> >> #INFORMATION LEAK
> >>
> >> In order to avoid this problem, we must serialize unlocked dio reads with
> >> truncate by inode_dio_wait().
> >>
> >
> > So I had thinking about this, are we sure we don't want to just lock the extent
> > range when we truncate? I'm good with this, but it seems like we might as well
> > and be consistent and use the extent locks. What do you think? Thanks,
>
> But comparing with the current approach, the extent lock has the following problem:
> Dio_Read_Task Truncate_task
> truncate file
> set isize to 4096
> drop pages
> lock extent[4096, 8191]
> read extent[4096, 8191]
> unlock extent[4096, 8191]
> lock extent[4096, -1ULL]
> truncate item
> unlock extent[4096, -1ULL]
> lock extent[8192, ...]
> read extent[8192, ...]
> no extent item
> zero the buffer
> unlock extent[8192, ...]
>
> we get the data that is mixed with new data.(Punch hole also has this problem, we need
> fix)
So this case is fine, since we'll still get valid data, the extents would still
be there. If you are mixing dio reads with simultaneous truncate/hole punching
you deserve to get your ass bitten :). The other option would be to lock before
we set the isize, or check the isize in get_extents. Thanks,
Josef
prev parent reply other threads:[~2013-02-01 14:40 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-31 9:23 [PATCH 1/2] Btrfs: serialize unlocked dio reads with truncate Miao Xie
2013-01-31 16:40 ` Josef Bacik
2013-02-01 5:56 ` Miao Xie
2013-02-01 14:40 ` Josef Bacik [this message]
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