linux-kernel - Re: [PATCH 2/8] xfs: Warn if direct reclaim tries to writeback pages

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Date:	Mon, 25 Jul 2011 09:19:18 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...hat.com>,
	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	XFS <xfs@....sgi.com>, Minchan Kim <minchan.kim@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Johannes Weiner <jweiner@...hat.com>
Subject: Re: [PATCH 2/8] xfs: Warn if direct reclaim tries to writeback pages

On Sun, Jul 24, 2011 at 07:32:00AM -0400, Christoph Hellwig wrote:
> On Thu, Jul 21, 2011 at 05:28:44PM +0100, Mel Gorman wrote:
> > --- a/fs/xfs/linux-2.6/xfs_aops.c
> > +++ b/fs/xfs/linux-2.6/xfs_aops.c
> > @@ -930,12 +930,13 @@ xfs_vm_writepage(
> >  	 * random callers for direct reclaim or memcg reclaim.  We explicitly
> >  	 * allow reclaim from kswapd as the stack usage there is relatively low.
> >  	 *
> > -	 * This should really be done by the core VM, but until that happens
> > -	 * filesystems like XFS, btrfs and ext4 have to take care of this
> > -	 * by themselves.
> > +	 * This should never happen except in the case of a VM regression so
> > +	 * warn about it.
> >  	 */
> > -	if ((current->flags & (PF_MEMALLOC|PF_KSWAPD)) == PF_MEMALLOC)
> > +	if ((current->flags & (PF_MEMALLOC|PF_KSWAPD)) == PF_MEMALLOC) {
> > +		WARN_ON_ONCE(1);
> >  		goto redirty;
> 
> The nicer way to write this is
> 
> 	if (WARN_ON(current->flags & (PF_MEMALLOC|PF_KSWAPD)) == PF_MEMALLOC)
> 		goto redirty;
> 

I wanted to avoid side effects if WARN_ON was compiled out similar to
the care that is normally taken for BUG_ON but it's unnecessary and
your version is far tidier. Do you really want WARN_ON used instead
of WARN_ON_ONCE()?

-- 
Mel Gorman
SUSE Labs
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