linux-kernel - Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

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Message-Id: <20070307013942.5c0fadff.akpm@linux-foundation.org>
Date:	Wed, 7 Mar 2007 01:39:42 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Bill Irwin <bill.irwin@...cle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Nick Piggin <npiggin@...e.de>,
	Linux Memory Management <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Paolo 'Blaisorblade' Giarrusso" <blaisorblade@...oo.it>
Subject: Re: [patch 4/6] mm: merge populate and nopage into fault (fixes
 nonlinear)

On Wed, 7 Mar 2007 01:29:03 -0800 Bill Irwin <bill.irwin@...cle.com> wrote:

> On Wed, 7 Mar 2007 09:27:55 +0100 Ingo Molnar <mingo@...e.hu> wrote:
> >> btw., if we decide that nonlinear isnt worth the continuing maintainance 
> >> pain, we could internally implement/emulate sys_remap_file_pages() via a 
> >> call to mremap() and essentially deprecate it, without breaking the ABI 
> >> - and remove all the nonlinear code. (This would split fremap areas into 
> >> separate vmas)
> 
> On Wed, Mar 07, 2007 at 12:35:20AM -0800, Andrew Morton wrote:
> > I'm rather regretting having merged it - I don't think it has been used for
> > much.
> > Paolo's UML speedup patches might use nonlinear though.
> 
> Guess what major real-life application not only uses nonlinear daily
> but would even be very happy to see it extended with non-vma-creating
> protections and more?

uh-oh.  SQL server?

> It's not terribly typical for things to be
> truncated while remap_file_pages() is doing its work, though it's been
> proposed as a method of dynamism. It won't stress remap_file_pages() vs.
> truncate() in any meaningful way, though, as userspace will be rather
> diligent about clearing in-use data out of the file offset range to be
> truncated away anyway, and all that via O_DIRECT.

The problem here isn't related to truncate or direct-IO.  It's just
plain-old MAP_SHARED.  nonlinear VMAs are now using the old-style
dirty-memory management.  msync() is basically a no-op and the code is
wildly tricky and pretty much untested.  The chances that we broke it are
considerable.

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