Re: [PATCH 6/6] writeback: merge for_kupdate and !for_kupdate cases

[Dave Chinner <david@fromorbit.com>]

On Sun, Jul 11, 2010 at 10:07:02AM +0800, Wu Fengguang wrote:
> Unify the logic for kupdate and non-kupdate cases.
> There won't be starvation because the inodes requeued into b_more_io
> will later be spliced _after_ the remaining inodes in b_io, hence won't
> stand in the way of other inodes in the next run.
> 
> It avoids unnecessary redirty_tail() calls, hence the update of
> i_dirtied_when. The timestamp update is undesirable because it could
> later delay the inode's periodic writeback, or exclude the inode from
> the data integrity sync operation (which will check timestamp to avoid
> extra work and livelock).
> 
> CC: Dave Chinner <david@fromorbit.com>
> Cc: Martin Bligh <mbligh@google.com>
> Cc: Michael Rubin <mrubin@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  fs/fs-writeback.c |   39 ++++++---------------------------------
>  1 file changed, 6 insertions(+), 33 deletions(-)
> 
> --- linux-next.orig/fs/fs-writeback.c	2010-07-11 09:13:32.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2010-07-11 09:13:36.000000000 +0800
> @@ -373,45 +373,18 @@ writeback_single_inode(struct inode *ino
>  		if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
>  			/*
>  			 * We didn't write back all the pages.  nfs_writepages()
> -			 * sometimes bales out without doing anything. Redirty
> -			 * the inode; Move it from b_io onto b_more_io/b_dirty.
> +			 * sometimes bales out without doing anything.
>  			 */
> -			/*
> -			 * akpm: if the caller was the kupdate function we put
> -			 * this inode at the head of b_dirty so it gets first
> -			 * consideration.  Otherwise, move it to the tail, for
> -			 * the reasons described there.  I'm not really sure
> -			 * how much sense this makes.  Presumably I had a good
> -			 * reasons for doing it this way, and I'd rather not
> -			 * muck with it at present.
> -			 */
> -			if (wbc->for_kupdate) {
> +			inode->i_state |= I_DIRTY_PAGES;
> +			if (wbc->nr_to_write <= 0) {
>  				/*
> -				 * For the kupdate function we move the inode
> -				 * to b_more_io so it will get more writeout as
> -				 * soon as the queue becomes uncongested.
> +				 * slice used up: queue for next turn
>  				 */
> -				inode->i_state |= I_DIRTY_PAGES;
> -				if (wbc->nr_to_write <= 0) {
> -					/*
> -					 * slice used up: queue for next turn
> -					 */
> -					requeue_io(inode);
> -				} else {
> -					/*
> -					 * somehow blocked: retry later
> -					 */
> -					redirty_tail(inode);
> -				}
> +				requeue_io(inode);
>  			} else {
>  				/*
> -				 * Otherwise fully redirty the inode so that
> -				 * other inodes on this superblock will get some
> -				 * writeout.  Otherwise heavy writing to one
> -				 * file would indefinitely suspend writeout of
> -				 * all the other files.
> +				 * somehow blocked: retry later
>  				 */
> -				inode->i_state |= I_DIRTY_PAGES;
>  				redirty_tail(inode);
>  			}

This means that congestion will always trigger redirty_tail(). Is
that really what we want for that case? Also, I'd prefer that the
comments remain somewhat more descriptive of the circumstances that
we are operating under. Comments like "retry later to avoid blocking
writeback of other inodes" is far, far better than "retry later"
because it has "why" component that explains the reason for the
logic. You may remember why, but I sure won't in a few months time....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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