linux-kernel - Re: per-bdi-throttling: synchronous writepage doesn't work correctly

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Date:	Thu, 01 Nov 2007 18:09:09 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	jdike@...toit.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: per-bdi-throttling: synchronous writepage doesn't work
	correctly

On Thu, 2007-11-01 at 18:00 +0100, Miklos Szeredi wrote:
> > > Hi,
> > > 
> > > It looks like bdi_thresh will always be zero if filesystem does
> > > synchronous writepage, resulting in very poor write performance.
> > > 
> > > Hostfs (UML) is one such example, but there might be others.
> > > 
> > > The only solution I can think of is to add a set_page_writeback();
> > > end_page_writeback() pair (or some reduced variant, that only does
> > > the proportions magic).  But that means auditing quite a few
> > > filesystems...
> > 
> > Ouch...
> > 
> > I take it there is no other function that is shared between all these
> > writeout paths which we could stick a bdi_writeout_inc(bdi) in?
> 
> No, and you can't detect it from the callers either I think.

The page not having PG_writeback set on return is a hint, but not fool
proof, it could be the device is just blazing fast.

I guess there is nothing to it but for me to grep writepage and manually
look at all hits...



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