linux-kernel - Re: [PATCH 0/7] writeback: avoid touching dirtied

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Date:	Fri, 21 Oct 2011 21:54:48 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/7] writeback: avoid touching dirtied_when on blocked
 inodes

On Fri 21-10-11 18:40:49, Wu Fengguang wrote:
> Hi Jan,
> 
> >   How about adding the attached patch to the series? With it applied we
> > would have all busyloop prevention done in the same way.
> 
> Sure. It looks good.
  Thanks.

> > +		pr_warn_ratelimited("mm: Possible busyloop in data writeback "
> > +			"(bdi %s nr_pages %ld sync_mode %d kupdate %d "
> > +			"background %d)\n",
> > +			wb->bdi->name, work->nr_pages, work->sync_mode,
> > +			work->for_kupdate, work->for_background);
> 
> I'll change the last two fields to the newly introduced writeback "reason":
> 
>                 pr_warn_ratelimited("mm: Possible busyloop in data writeback "
>                         "(bdi %s nr_pages %ld sync_mode %d reason %s)\n",
>                         wb->bdi->name, work->nr_pages, work->sync_mode,
>                         wb_reason_name[work->reason]);
  Makes sense. Thanks.

> btw, with the I_SYNC case converted, it's actually no longer necessary
> to keep a standalone b_more_io_wait. It should still be better to keep
> the list and the above error check for catching possible errors and
> the flexibility of adding policies like "don't retry possible blocked
> inodes in N seconds as long as there are other inodes to work with".
> 
> The below diff only intends to show the _possibility_ to remove
> b_more_io_wait:
  Good observation. So should we introduce b_more_io_wait in the end? We
could always introduce it when the need for some more complicated policy
comes...

								Honza

> 
> --- linux-next.orig/fs/fs-writeback.c	2011-10-21 18:25:25.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2011-10-21 18:27:41.000000000 +0800
> @@ -235,20 +235,7 @@ static void requeue_io(struct inode *ino
>  	list_move(&inode->i_wb_list, &wb->b_more_io);
>  }
>  
> -/*
> - * The inode should be retried in an opportunistic way.
> - *
> - * The only difference between b_more_io and b_more_io_wait is:
> - * wb_writeback() won't quit as long as b_more_io is not empty.  When
> - * wb_writeback() quit on empty b_more_io and non-empty b_more_io_wait,
> - * the kupdate work will wakeup more frequently to retry the inodes in
> - * b_more_io_wait.
> - */
> -static void requeue_io_wait(struct inode *inode, struct bdi_writeback *wb)
> -{
> -	assert_spin_locked(&wb->list_lock);
> -	list_move(&inode->i_wb_list, &wb->b_more_io_wait);
> -}
> +#define requeue_io_wait(inode, wb) requeue_io(inode, wb)
>  
>  static void inode_sync_complete(struct inode *inode)
>  {
> @@ -798,21 +785,8 @@ static long wb_writeback(struct bdi_writ
>  		 * mean the overall work is done. So we keep looping as long
>  		 * as made some progress on cleaning pages or inodes.
>  		 */
> -		if (progress)
> -			continue;
> -		/*
> -		 * No more inodes for IO, bail
> -		 */
> -		if (list_empty(&wb->b_more_io))
> +		if (!progress)
>  			break;
> -		/*
> -		 * Nothing written but some inodes were moved to b_more_io.
> -		 * This should not happen as we can easily busyloop.
> -		 */
> -		pr_warn_ratelimited("mm: Possible busyloop in data writeback "
> -			"(bdi %s nr_pages %ld sync_mode %d reason %s)\n",
> -			wb->bdi->name, work->nr_pages, work->sync_mode,
> -			wb_reason_name[work->reason]);
>  	}
>  	spin_unlock(&wb->list_lock);
> 
> Thanks,
> Fengguang
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