linux-kernel - Re: [PATCH 07/11] writeback: periodically trim the writeback list

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Date:	Thu, 1 Aug 2013 16:16:18 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, davej@...hat.com,
	viro@...iv.linux.org.uk, glommer@...allels.com
Subject: Re: [PATCH 07/11] writeback: periodically trim the writeback list

On Wed, Jul 31, 2013 at 05:15:42PM +0200, Jan Kara wrote:
> On Wed 31-07-13 14:15:46, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@...hat.com>
> > 
> > Inodes are removed lazily from the bdi writeback list, so in the
> > absence of sync(2) work inodes will build up on the bdi writback
> > list even though they are no longer under IO. Use the periodic
> > kupdate work check to remove inodes no longer under IO from the
> > writeback list.
> > 
> > Signed-off-by: Dave Chinner <dchinner@...hat.com>
> > ---
> >  fs/fs-writeback.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> > 
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 638f122..7c9bbf0 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -962,6 +962,23 @@ static long wb_check_background_flush(struct bdi_writeback *wb)
> >  	return 0;
> >  }
> >  
> > +/*
> > + * clean out writeback list for all inodes that don't have IO in progress
> > + */
> > +static void wb_trim_writeback_list(struct bdi_writeback *wb)
> > +{
> > +	struct inode *inode;
> > +	struct inode *tmp;
> > +
> > +	spin_lock(&wb->list_lock);
> > +	list_for_each_entry_safe(inode, tmp, &wb->b_writeback, i_wb_list) {
> > +		if (!mapping_tagged(inode->i_mapping, PAGECACHE_TAG_WRITEBACK))
> > +			list_del_init(&inode->i_wb_list);
> > +	}
>    Oo, and here you manipulate i_wb_list without mapping->tree_lock so that
> can race with the list_empty() check in bdi_mark_inode_writeback().

I'm not sure it does - we only remove is the PAGECACHE_TAG_WRITEBACK
is not set, and only insert after we set the tag. Hence, if we are
walking the &wb->b_writeback list here and PAGECACHE_TAG_WRITEBACK
is set because there is an insert in progress then we can't race
with the insert because we won't be trying to delete it from the
list...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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