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Subject: Re: [PATCH] nfs: fix nfs_writepage()
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
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Date: Wed, 17 Oct 2007 12:24:13 -0400
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On Wed, 2007-10-17 at 17:57 +0200, Peter Zijlstra wrote:
> On Wed, 2007-10-17 at 11:47 -0400, Trond Myklebust wrote:
> > On Wed, 2007-10-17 at 11:45 -0400, Trond Myklebust wrote:
> > > On Wed, 2007-10-17 at 17:30 +0200, Peter Zijlstra wrote:
> > > 
> > > > The thing is, swapper_space just calls ->writepage() and expects the
> > > > page to be written out. So either the a_ops usage of swapper_space is
> > > > deviant or NFS' is.
> > > 
> > > Could somebody please document WTF writepage() is supposed to do, and
> > > WTF page_mkwrite() is for?
> > > 
> > > I thought that page_mkwrite() was supposed to finally allow us to deal
> > > with dirty pages in a clean manner: the caller gets to tell the
> > > filesystem that it wants the entire page written out, and then dirties
> > > the page. What is the point if the VM then expects to be able to
> > > circumvent this?
> > 
> > Put differently:
> >       * _who_ is dirtying the page when the swapper is trying to write
> >         the page out?
> >       * why are they not calling either page_mkwrite() or
> >         commit_write()?
> 
> I'm writing anonymous pages (I'm the crazy person doing swap over NFS).
> And anonymous is dirty by default.

I'd really prefer to _know_ that these writes are coming from the
swapper. That makes it possible to give them a correct credential and
open context.

Could you perhaps funnel them through a new a_op->swap_out() and default
back to ->writepage() for those filesystems that don't define it?