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

[Jan Kara <jack@suse.cz>]

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.

> >  		} 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.

								Honza
> 
> > +			else
> > +				requeue_io(inode, wb);
> >  		} else {
> >  			/*
> >  			 * The inode is clean.  At this point we either have
> 
> Thanks,
> Fengguang
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR