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Date: Thu, 1 Jan 2004 23:31:48 -0800
From: Andrew Morton <akpm@osdl.org>
To: suparna@in.ibm.com
Cc: daniel@osdl.org, janetmor@us.ibm.com, pbadari@us.ibm.com,
       linux-aio@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch
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Suparna Bhattacharya <suparna@in.ibm.com> wrote:
>
> On Wed, Dec 31, 2003 at 03:17:36AM -0800, Andrew Morton wrote:
> > Andrew Morton <akpm@osdl.org> wrote:
> > >
> > > Let me actually think about this a bit.
> > 
> > Nasty.  The same race is present in 2.4.x...
> > 
> > How's about we start new I/O in filemap_fdatawait() if the page is dirty?
> > 
> 
> Makes sense to me.
> There's a chance that this could explain why Daniel saw exposures even 
> with his fix. 
> 
> Would be interesting to see his results with your patch.
> 
> Though we might as well plug this anyway ?

Yes, we should.  I'm not dreadfully keen on starting I/O from within
filemap_fdatawait() though - it seems "wrong" somehow.

> > 
> > diff -puN mm/filemap.c~a mm/filemap.c
> > --- 25/mm/filemap.c~a	2003-12-31 03:10:29.000000000 -0800
> > +++ 25-akpm/mm/filemap.c	2003-12-31 03:17:05.000000000 -0800
> > @@ -206,7 +206,13 @@ restart:
> >  		page_cache_get(page);
> >  		spin_unlock(&mapping->page_lock);
> >  
> > -		wait_on_page_writeback(page);
> > +		lock_page(page);
> > +		if (PageDirty(page) && mapping->a_ops->writepage) {
> > +			write_one_page(page, 1);
> > +		} else {
> > +			wait_on_page_writeback(page);
> > +			unlock_page(page);
> 
> Would we lose anything if we unlock_page() before wait_on_page_writeback() ?

No, that should be OK.