Re: [PATCH 6/7] ext4: endless truncate due to nonlocked dio readers V2

[Jan Kara <jack@suse.cz>]

  Hello,

On Thu 13-09-12 18:41:36, Zheng Liu wrote:
> Could you please provide more detailed workload to convince me?  I
> am thinking about whether we really need to disable dioread_nolock
> feature in here.  In our benchmarks, we don't see this problem.
  I just did:

# Create file
dd if=/dev/zero of=/mnt/file bs=1M count=30
sync
# Start 10 DIO dio readers in parallel reading the file in a loop
for (( i = 0; i < 10; i++ )); do
  while true; do
    dd if=/mnt/file bs=4k iflag=direct of=/dev/null
  done &
done
sleep 1

# Try to truncate the file - never finishes.
truncate -s 16 /mnt/file

It is pretty easy to hit this. Besides being a DOS attack vector (but I
won't be too concerned about this - there are plenty of ways how local
process can screw you) I can easily imagine some application to get bitten
by this. 

								Honza
> 
> Regards,
> Zheng
> 
> On Sun, Sep 09, 2012 at 09:27:13PM +0400, Dmitry Monakhov wrote:
> > If we have enough aggressive DIO readers, truncate and other dio
> > waiters will wait forever inside inode_dio_wait(). It is reasonable
> > to disable nonlock DIO read optimization during truncate.
> > 
> > Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
> > ---
> >  fs/ext4/extents.c |    2 +-
> >  fs/ext4/fsync.c   |    2 +-
> >  fs/ext4/inode.c   |    2 +-
> >  3 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 8252651..b5b801f 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -4853,7 +4853,7 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
> >  	}
> >  
> >  	/* finish any pending end_io work */
> > -	inode_dio_wait(inode);
> > +	ext4_inode_dio_wait(inode, 1);
> >  	ext4_flush_completed_IO(inode);
> >  
> >  	credits = ext4_writepage_trans_blocks(inode);
> > diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
> > index 290c5cf..bdf6bfd 100644
> > --- a/fs/ext4/fsync.c
> > +++ b/fs/ext4/fsync.c
> > @@ -204,7 +204,7 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
> >  	if (inode->i_sb->s_flags & MS_RDONLY)
> >  		goto out;
> >  
> > -	inode_dio_wait(inode);
> > +	ext4_inode_dio_wait(inode, 1);
> >  	ret = ext4_flush_completed_IO(inode);
> >  	if (ret < 0)
> >  		goto out;
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 93e6b09..a850026 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -4335,7 +4335,7 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
> >  			truncate_setsize(inode, attr->ia_size);
> >  			/* Inode size will be reduced, wait for dio in flight */
> >  			if (orphan)
> > -				inode_dio_wait(inode);
> > +				ext4_inode_dio_wait(inode, 1);
> >  		}
> >  		ext4_truncate(inode);
> >  	}
> > -- 
> > 1.7.7.6
> > 
> > --
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-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR