linux-kernel - Re: [PATCH 02/17] writeback: update dirtied_when for synced inode to prevent livelock

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Date:	Mon, 23 May 2011 15:14:24 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/17] writeback: update dirtied_when for synced inode
 to prevent livelock

On Fri 20-05-11 05:31:19, Wu Fengguang wrote:
> > > > @@ -419,6 +419,15 @@ writeback_single_inode(struct inode *ino
> > > >  	spin_lock(&inode->i_lock);
> > > >  	inode->i_state &= ~I_SYNC;
> > > >  	if (!(inode->i_state & I_FREEING)) {
> > > > +		/*
> > > > +		 * Sync livelock prevention. Each inode is tagged and synced in
> > > > +		 * one shot, so we can unconditionally update its dirty time to
> > > > +		 * prevent syncing it again. Note that time ordering of b_dirty
> > > > +		 * list will be kept because the following code either removes
> > > > +		 * the inode from b_dirty or calls redirty_tail().
> > > > +		 */
> > > > +		if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_sync)
> > > > +			inode->dirtied_when = jiffies;
> > > 
> > > Shouldn't this  update only ocur if the inode is still dirty?
> > 
> > Yeah, that would be better even though the current form won't lead to
> > errors.
> > 
> > Let's add one more test (inode->i_state & I_DIRTY)?
> > (I was actually aware of the trade offs and didn't bother to add it..)
> 
> Oops, the (inode->i_state & I_DIRTY) test is not enough because
> when the inode is actively dirtied, I_DIRTY_PAGES won't be set when
> the flusher is writing out the pages with I_SYNC set...
  Well, rather on contrary I_DIRTY_PAGES won't be set when noone dirtied
any page after we cleared I_DIRTY_PAGES.

> Well it would look clumsy to add another mapping_tagged(mapping,
> PAGECACHE_TAG_DIRTY) test. So I tend to revert to the original scheme
> of updating ->dirtied_when only on newly dirtied pages. It's the
> simplest form that can avoid unnecessarily polluting ->dirtied_when.
  Hmm, but won't now something like:
while true; do touch f; done
  livelock sync? We always manage to write something - 1 inode - and the
inode will never be clean (before the IO completes, the other process
manages to dirty the inode again with very high probability).

								Honza
> 
> @@ -432,6 +432,15 @@ writeback_single_inode(struct inode *ino
>  				requeue_io(inode);
>  			} else {
>  				/*
> +				 * Sync livelock prevention. Each inode is
> +				 * tagged and synced in one shot, so we can
> +				 * unconditionally update its dirty time to
> +				 * prevent syncing it again.
> +				 */
> +				if (wbc->sync_mode == WB_SYNC_ALL ||
> +				    wbc->tagged_writepages)
> +					inode->dirtied_when = jiffies;
> +				/*
>  				 * Writeback blocked by something other than
>  				 * congestion. Delay the inode for some time to
>  				 * avoid spinning on the CPU (100% iowait)
> 
> Thanks,
> Fengguang
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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