Re: [PATCH 2/2] writeback: Replace some redirty_tail() calls with requeue_io()

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Sep 08, 2011 at 11:03:40PM +0800, Jan Kara wrote:
> On Thu 08-09-11 09:22:36, Wu Fengguang wrote:
> > Jan,
> > 
> > > @@ -420,6 +421,8 @@ writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
> > >  	/* Don't write the inode if only I_DIRTY_PAGES was set */
> > >  	if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
> > >  		int err = write_inode(inode, wbc);
> > > +		if (!err)
> > > +			inode_written = true;
> > >  		if (ret == 0)
> > >  			ret = err;
> > >  	}
> > 
> > write_inode() typically return error after redirtying the inode.
> > So the conditions inode_written=false and (inode->i_state & I_DIRTY)
> > are mostly on/off together. For the cases they disagree, it's probably
> > a filesystem bug -- at least I don't think some FS will deliberately 
> > return success while redirtying the inode, or the reverse.
>   There is a possibility someone else redirties the inode between the moment
> I_DIRTY bits are cleared in writeback_single_inode() and the check for
> I_DIRTY is done after ->write_inode() is called. Especially when
> write_inode() blocks waiting for some IO this isn't that hard to happen. So
> there are valid (although relatively rare) cases when inode_written is
> different from the result of I_DIRTY check.

Ah yes, that's good point.

> > >  		} else if (inode->i_state & I_DIRTY) {
> > >  			/*
> > >  			 * Filesystems can dirty the inode during writeback
> > >  			 * operations, such as delayed allocation during
> > >  			 * submission or metadata updates after data IO
> > > -			 * completion.
> > > +			 * completion. Also inode could have been dirtied by
> > > +			 * some process aggressively touching metadata.
> > > +			 * Finally, filesystem could just fail to write the
> > > +			 * inode for some reason. We have to distinguish the
> > > +			 * last case from the previous ones - in the last case
> > > +			 * we want to give the inode quick retry, in the
> > > +			 * other cases we want to put it back to the dirty list
> > > +			 * to avoid livelocking of writeback.
> > >  			 */
> > > -			redirty_tail(inode, wb);
> > > +			if (inode_written)
> > > +				redirty_tail(inode, wb);
> > 
> > Can you elaborate the livelock in the below inode_written=true case?
> > Why the sleep in the wb_writeback() loop is not enough?
>   In case someone would be able to consistently trigger the race window and
> redirty the inode before we check here, we would loop for a long time
> always writing just this inode and thus effectivelly stalling other
> writeback. That's why I push redirtied inode behind other inodes in the
> dirty list.

Agreed. All the left to do is to confirm whether this addresses
Christoph's original problem.

Acked-by: Wu Fengguang <fengguang.wu@intel.com>

Thanks,
Fengguang