linux-kernel - Re: [rfc] optimise unlock

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Message-ID: <Pine.LNX.4.64.0705161737140.7914@blonde.wat.veritas.com>
Date:	Wed, 16 May 2007 18:21:09 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Nick Piggin <npiggin@...e.de>
cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [rfc] optimise unlock_page

On Sun, 13 May 2007, Nick Piggin wrote:
> On Fri, May 11, 2007 at 02:15:03PM +0100, Hugh Dickins wrote:
> 
> > But again I wonder just what the gain has been, once your double
> > unmap_mapping_range is factored in.  When I suggested before that
> > perhaps the double (well, treble including the one in truncate.c)
> > unmap_mapping_range might solve the problem you set out to solve
> > (I've lost sight of that!) without pagelock when faulting, you said:
> > 
> > > Well aside from being terribly ugly, it means we can still drop
> > > the dirty bit where we'd otherwise rather not, so I don't think
> > > we can do that.
> > 
> > but that didn't give me enough information to agree or disagree.
> 
> Oh, well invalidate wants to be able to skip dirty pages or have the
> filesystem do something special with them first. Once you have taken
> the page out of the pagecache but still mapped shared, then blowing
> it away doesn't actually solve the data loss problem... only makes
> the window of VM inconsistency smaller.

Right, I think I see what you mean now, thanks: userspace
must not for a moment be allowed to write to orphaned pages.

Whereas it's not an issue for the privately COWed pages you added
the second unmap_mapping_range for: because it's only truncation
that has to worry about them, so they're heading for SIGBUS anyway.

Yes, and the page_mapped tests in mm/truncate.c are just racy
heuristics without the page lock you now put into faulting.

Hugh
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