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

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

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.

>  		} 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?

> +			else
> +				requeue_io(inode, wb);
>  		} else {
>  			/*
>  			 * The inode is clean.  At this point we either have

Thanks,
Fengguang