linux-kernel - Re: [PATCH 1/6] writeback: take account of NR_WRITEBACK_TEMP in balance_dirty

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Message-ID: <20100715145045.GA6511@localhost>
Date:	Thu, 15 Jul 2010 22:50:45 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"richard@....demon.co.uk" <richard@....demon.co.uk>,
	"david@...morbit.com" <david@...morbit.com>,
	"jack@...e.cz" <jack@...e.cz>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] writeback: take account of NR_WRITEBACK_TEMP in
 balance_dirty_pages()

On Tue, Jul 13, 2010 at 04:58:47PM +0800, Miklos Szeredi wrote:
> On Mon, 12 Jul 2010, Andrew Morton wrote:
> > On Sun, 11 Jul 2010 10:06:57 +0800
> > Wu Fengguang <fengguang.wu@...el.com> wrote:
> > 
> > > 
> > > Signed-off-by: Richard Kennedy <richard@....demon.co.uk>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > > ---
> > >  mm/page-writeback.c |    7 ++++---
> > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > > 
> > > --- linux-next.orig/mm/page-writeback.c	2010-07-11 08:41:37.000000000 +0800
> > > +++ linux-next/mm/page-writeback.c	2010-07-11 08:42:14.000000000 +0800
> > > @@ -503,11 +503,12 @@ static void balance_dirty_pages(struct a
> > >  		};
> > >  
> > >  		get_dirty_limits(&background_thresh, &dirty_thresh,
> > > -				&bdi_thresh, bdi);
> > > +				 &bdi_thresh, bdi);
> > >  
> > >  		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> > > -					global_page_state(NR_UNSTABLE_NFS);
> > > -		nr_writeback = global_page_state(NR_WRITEBACK);
> > > +				 global_page_state(NR_UNSTABLE_NFS);
> > > +		nr_writeback = global_page_state(NR_WRITEBACK) +
> > > +			       global_page_state(NR_WRITEBACK_TEMP);
> > >  
> > >  		bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> > >  		bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
> > > 
> > 
> > hm, OK.
> 
> Hm, hm.  I'm not sure this is right.  The VM has absolutely no control
> over NR_WRITEBACK_TEMP pages, they may clear quickly or may not make
> any progress.  So it's usually wrong to make a decision based on
> NR_WRITEBACK_TEMP for an unrelated device.

Ah OK, let's remove this patch.

> Using it in throttle_vm_writeout() would actually be deadlocky, since
> the userspace filesystem will probably depend on memory allocations to
> complete the writeout.

Right.

> The only place where we should be taking NR_WRITEBACK_TEMP into
> account is calculating the remaining memory that can be devided
> between dirtyers, and that's (clip_bdi_dirty_limit) where it is
> already used.

clip_bdi_dirty_limit() is removed in the next patch, hopefully it's OK.

> > I wonder whether we could/should have unified NR_WRITEBACK_TEMP and
> > NR_UNSTABLE_NFS.  Their "meanings" aren't quite the same, but perhaps
> > some "treat page as dirty because the fs is futzing with it" thing.
> 
> AFAICS NR_UNSTABLE_NFS is something akin to NR_DIRTY, only on the
> server side.  So nfs can very much do something about making
> NR_UNSTABLE_NFS go away, while there's nothing that can be done about
> NR_WRITEBACK_TEMP.

Right. nfs_write_inode() normally tries to commit unstable pages.

Thanks,
Fengguang
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