linux-ext4 - Re: strange ext{3,4}

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Date:	Mon, 17 Mar 2008 09:24:15 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Dmitri Monakhov <dmonakhov@...nvz.org>
Cc:	Andreas Dilger <adilger@....com>, linux-ext4@...r.kernel.org
Subject: Re: strange ext{3,4}_settattr logic

On Sun, Mar 16 2008, Dmitri Monakhov wrote:
> I've added Jens because he may be also interesting in this topic.
> On 08:23 Sun 16 Mar     , Andreas Dilger wrote:
> > On Mar 16, 2008  07:54 +0800, Andreas Dilger wrote:
> > > A call to  inode_setattr() can fail by trying a shrinking vmtruncate()
> > > on a swapfile, which returns ETXTBUSY.  This was added after the
> > > ext3_setattr() code was written.
> > > 
> > > We need to handle the IS_SWAPFILE() case properly.
> > > Granted, it probably isn't a very common problem, but the IS_SWAPFILE()
> > > check was added explicitly because of clueless users, so it must be hit
> > > occasionally in real life.
> > > 
> > > It would seem that if you have a swapfile, try to truncate it to 0 (which
> > > will fail with -ETXTBUSY) and then unmount the filesystem the size will
> > > be truncated to 0.  It is also possible to directly write to a swapfile
> > > and corrupt memory, or read from a swapfile and access potentially sensitive
> > > information.
> In fact i've triggered this issue while working on fast_loop device
> implementation which was proposed by Jens (http://lkml.org/lkml/2008/1/9/50).
> In fast_loop device use swapfile approach (submitting bio-s directly to
> underlying block device). I think this idea will be included in mainstream
> loop device sooner or later. But approach has several issues:
> One of the most important is effective control mechanism over truncates for
> lower file,  this issue was missed in Jens patch set.
> This mechanism probably have to have following options.
> #1: Shrink truncates must be denied.
> #2: Expand truncates may be allowed. This is good because most of non plain 
> disk image formats (qcow, vmdk, and etc) are growing while adding new data
> blocks.
> #3: Allow exclusive owner for file, for example only one user(loop_thread in
> this case) may truncate file. Provide something similar to bd_claim feature.
> without this feature on-line shrinking of disk image looks like this:
> 
>  mutex_lock(&inode->i_mutex);
>  inode->i_flags &= ~S_SWAPFILE;
>  mutex_unlock(&inode->i_mutex);
> 
>  /* Perform shrinking truncate. This is absolutely racy operation because
>   * some one else also may perform truncate at this time*/
>  do_truncate(inode, size); 
> 
>  mutex_lock(&inode->i_mutex);
>  inode->i_flags |= S_SWAPFILE;
>  mutex_unlock(&inode->i_mutex);
> 
> 
> S_SWAPFILE inode option currently equals to #1, and #2. What's why i want
> use this flag for fast_loop devices.

Neat, I didn't consider that. Mainly because I had (knowingly) ignored
the exclusive owner bit so far. I have included your suggestion in the
loop-fastfs and loop-extent_map branches, thanks.

-- 
Jens Axboe

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