On Mon, May 03, 2010 at 09:53:44AM +0530, Nikanth Karthikesan wrote:
quoted text
> On Saturday 01 May 2010 12:34:26 Amit K. Arora wrote:
> > On Fri, Apr 30, 2010 at 02:33:19PM -0700, Andrew Morton wrote:
> > > Also, there doesn't seem to be much point in doing
> > >
> > > 	mutex_lock(i_mutex);
> > > 	if (some_condition)
> > > 		bale out
> > > 	mutex_unlock(i_mutex);
> > >
> > > 	<stuff>
> > >
> > > because `some_condition' can now become true before or during the
> > > execution of `stuff'.
> > >
> > > IOW, it's racy.
> 
> oh, yes. :(
> 
> > Agreed. How about doing this check in the filesystem specific fallocate
> > inode routines instead ? For example, in ext4 we could do :
> 
> I guess, calling the filesystem specific fallocate with the lock held would 
> create lock ordering problems? If so, this might be the only way. But it would 
> be better to document at the call site, that the callee should check for 
> RLIMIT_FSIZE.

Hmm.. I never said to call the filesystem specific fallocate with
i_mutex held. What I suggested was that each filesystem at some point
anyhow takes the i_mutex to preallocate. Thats where the check should
be, to avoid the race. This is what the example patch below does.

--
Regards,
Amit Arora

quoted text
> Thanks
> Nikanth
> 
> > diff -Nuarp linux-2.6.org/fs/ext4/extents.c linux-2.6.new/fs/ext4/extents.c
> > --- linux-2.6.org/fs/ext4/extents.c	2010-05-01 12:16:07.000000000 +0530
> > +++ linux-2.6.new/fs/ext4/extents.c	2010-05-01 12:17:37.000000000 +0530
> > @@ -3672,6 +3672,11 @@ long ext4_fallocate(struct inode *inode,
> >  	 */
> >  	credits = ext4_chunk_trans_blocks(inode, max_blocks);
> >  	mutex_lock(&inode->i_mutex);
> > +	ret = inode_newsize_ok(inode, (offset + len));
> > +	if (ret) {
> > +		mutex_unlock(&inode->i_mutex);
> > +		return ret;
> > +	}
> >  retry:
> >  	while (ret >= 0 && ret < max_blocks) {
> >  		block = block + ret;
> > 
> > 
> > Similarly for ocfs2, btrfs and xfs..
> > 
> > --
> > Regards,
> > Amit Arora
> > 
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