Re: [PATCH 1/4] ext4: serialize dio nonlocked reads with defrag workers

[Dmitry Monakhov <dmonakhov@openvz.org>]

On Wed, 5 Sep 2012 17:47:19 +0200, Jan Kara <jack@suse.cz> wrote:
> On Tue 04-09-12 21:36:51, Dmitry Monakhov wrote:
> > Inode's block defrag and ext4_change_inode_journal_flag() may
> > affect nonlocked DIO reads result, so proper synchronization
> > required.
> > 
> > - add missed inode_dio_wait() calls where appropriate
> > - recheck ext4_should_dioread_nolock under extra i_dio_count reference.
> > 
> > Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
> ...
> > diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> > index 830e1b2..ba40309 100644
> > --- a/fs/ext4/indirect.c
> > +++ b/fs/ext4/indirect.c
> > @@ -812,10 +812,22 @@ retry:
> >  			ext4_flush_completed_IO(inode);
> >  			mutex_unlock(&inode->i_mutex);
> >  		}
> > +		/*
> > +		 * Inode's locking behaviour may change due to number
> > +		 * of reasons, in order to be shure that nolock dioreads
> 					      ^^^ sure
> > +		 * is still allowed we have to recheck inode's flags
> > +		 * while i_dio_count > 0
> > +		 */
> > +		atomic_inc(&inode->i_dio_count);
> > +		if (!unlikely(ext4_should_dioread_nolock(inode))) {
> > +			inode_dio_done(inode);
> > +			goto retry;
> > +		}
>   Umm, to make this reliable, you need a smp_mb() between atomic_inc() and
> ext4_should_dioread_nolock(). Otherwise the i_state test could be reordered
> before the atomic_inc()... Similarly you need the barriers around all other
> pairs working with i_dio_count and i_state.
Ohh, you right. It looks reasonable to grab down_read(i_data_sem) here.
> 
> Frankly, I'm not very happy with this solution. It's getting ugly rather
> quickly. IMHO dioread_nolock should be just ripped out and replaced with
> proper range locking (if we want the scalability). But this is mostly Ted's
> decision...
> 
> 								Honza
> 
> >  		ret = __blockdev_direct_IO(rw, iocb, inode,
> >  				 inode->i_sb->s_bdev, iov,
> >  				 offset, nr_segs,
> >  				 ext4_get_block, NULL, NULL, 0);
> > +		inode_dio_done(inode);
> >  	} else {
> >  		ret = blockdev_direct_IO(rw, iocb, inode, iov,
> >  				 offset, nr_segs, ext4_get_block);
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index d12d30e..58ef61a 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -4741,6 +4741,10 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
> >  			return err;
> >  	}
> >  
> > +	/* Wait for all existing dio workers */
> > +	ext4_set_inode_state(inode, EXT4_STATE_DIOREAD_LOCK);
> > +	inode_dio_wait(inode);
> > +
> >  	jbd2_journal_lock_updates(journal);
> >  
> >  	/*
> > @@ -4760,6 +4764,7 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
> >  	ext4_set_aops(inode);
> >  
> >  	jbd2_journal_unlock_updates(journal);
> > +	ext4_clear_inode_state(inode, EXT4_STATE_DIOREAD_LOCK);
> >  
> >  	/* Finally we can mark the inode as dirty. */
> >  
> > diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
> > index c5826c6..a6a4278 100644
> > --- a/fs/ext4/move_extent.c
> > +++ b/fs/ext4/move_extent.c
> > @@ -1213,6 +1213,14 @@ ext4_move_extents(struct file *o_filp, struct file *d_filp,
> >  	ret1 = mext_inode_double_lock(orig_inode, donor_inode);
> >  	if (ret1 < 0)
> >  		return ret1;
> > +	/* Protect inodes against DIO workers
> > +	 * - Disable dio nonlock reads, so all new dio workers will block
> > +	 *   on i_mutex.
> > +	 * - wait for existing DIO in flight */
> > +	ext4_set_inode_state(orig_inode, EXT4_STATE_DIOREAD_LOCK);
> > +	ext4_set_inode_state(donor_inode, EXT4_STATE_DIOREAD_LOCK);
> > +	inode_dio_wait(orig_inode);
> > +	inode_dio_wait(donor_inode);
> >  
> >  	/* Protect extent tree against block allocations via delalloc */
> >  	double_down_write_data_sem(orig_inode, donor_inode);
> > @@ -1412,6 +1420,8 @@ out:
> >  		kfree(holecheck_path);
> >  	}
> >  	double_up_write_data_sem(orig_inode, donor_inode);
> > +	ext4_clear_inode_state(orig_inode, EXT4_STATE_DIOREAD_LOCK);
> > +	ext4_clear_inode_state(donor_inode, EXT4_STATE_DIOREAD_LOCK);
> >  	ret2 = mext_inode_double_unlock(orig_inode, donor_inode);
> >  
> >  	if (ret1)
> > -- 
> > 1.7.7.6
> > 
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR
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