linux-ext4 - Re: [PATCH] ext4: Prevent race while waling extent tree

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Date:	Thu, 8 Nov 2012 14:43:19 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	tytso@....edu
Subject: Re: [PATCH] ext4: Prevent race while waling extent tree

On Thu, 8 Nov 2012, Dmitry Monakhov wrote:

> Date: Thu, 08 Nov 2012 16:01:17 +0400
> From: Dmitry Monakhov <dmonakhov@...nvz.org>
> To: Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org
> Cc: tytso@....edu, Lukas Czerner <lczerner@...hat.com>
> Subject: Re: [PATCH] ext4: Prevent race while waling extent tree
> 
> On Thu,  8 Nov 2012 12:08:49 +0100, Lukas Czerner <lczerner@...hat.com> wrote:
> > Currently ext4_ext_walk_space() only takes i_data_sem for read when
> > searching for the extent at given block with ext4_ext_find_extent().
> > Then it drops the lock and the extent tree can be changed at will.
> > However later on we're searching for the 'next' extent, but the extent
> > tree might already have changed, so the information might not be
> > accurate.
> > 
> > In fact we can hit BUG_ON(end <= start) if the extent got inserted into
> > the tree after the one we found and before the block we were searching
> > for. This has been reproduced by running xfstests 225 in loop on s390x
> > architecture, but theoretically we could hit this on any other
> > architecture as well, but probably not as often.
> > 
> > ext4_ext_walk_space() is currently only used from ext4_fiemap() and even
> > if we do not hit the BUG_ON() fiemap might return scrambled information
> > to the user.
> > 
> > Fix this by requiring ext4_ext_walk_space() to be called with i_data_sem
> > held. By calling it from ext4_fiemap() we can only take the i_data_sem
> > for read, but possibly other users might want to modify the extents so
> > they will be able to take write lock.
> Agree as a short term fix for BUGON case, but Theodore suggested to use
> seqlock approach http://lists.openwall.net/linux-ext4/2011/10/26/25

Yeah, it make sense to protect us from fiemap abuse, however using
seqlock for walking the extent tree seems like an overkill
especially considering how much work will that require. We would
have to make sure that everything we do in the ext4_ext_walk_space()
and other function we're calling there is safe even if the extent
tree change under our hands. I do not think this is the right way.

I was thinking about checking for contentions on the semaphore from
within the ext4_ext_walk_space() - possibly enabling/disabling it
with a function parameter ?

Sadly kernel does not provide a helper to check for that so what
about something like this in the beginning of the while loop in
ext4_ext_walk_space ?

if (check_contention) {
	int contends = 0;
	unsigned int flags;

	raw_spin_lock_irqsave(&EXT4_I(inode)->i_data_sem->wait_lock, flags);
	if (!list_empty(&EXT4_I(inode)->i_data_sem->wait_list)
		contends = 1
	raw_spin_unlock_irqrestore(&EXT4_I(inode)->i_data_sem->wait_lock, flags);

	if (contends)
		break
}

or we can add the helper to the rwsem code and use that.


What do you think ?

Thanks!
-Lukas

> 
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > ---
> >  fs/ext4/extents.c |    9 +++++++--
> >  1 files changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 7011ac9..f1aca06 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -1959,6 +1959,11 @@ cleanup:
> >  	return err;
> >  }
> >  
> > +/*
> > + * ext4_ext_walk_space() should be called with i_data_sem locked. If we're
> > + * not modifying found extents, or extent tree in callback function, then
> > + * read lock is ok.
> > + */
> >  static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
> >  			       ext4_lblk_t num, ext_prepare_callback func,
> >  			       void *cbdata)
> > @@ -1976,9 +1981,7 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
> >  	while (block < last && block != EXT_MAX_BLOCKS) {
> >  		num = last - block;
> >  		/* find extent for this block */
> > -		down_read(&EXT4_I(inode)->i_data_sem);
> >  		path = ext4_ext_find_extent(inode, block, path);
> > -		up_read(&EXT4_I(inode)->i_data_sem);
> >  		if (IS_ERR(path)) {
> >  			err = PTR_ERR(path);
> >  			path = NULL;
> > @@ -5021,8 +5024,10 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> >  		 * Walk the extent tree gathering extent information.
> >  		 * ext4_ext_fiemap_cb will push extents back to user.
> >  		 */
> > +		down_read(&EXT4_I(inode)->i_data_sem);
> >  		error = ext4_ext_walk_space(inode, start_blk, len_blks,
> >  					  ext4_ext_fiemap_cb, fieinfo);
> > +		up_read(&EXT4_I(inode)->i_data_sem);
> >  	}
> >  
> >  	return error;
> > -- 
> > 1.7.7.6
> > 
> > --
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