linux-kernel - Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young

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Date:	Wed, 27 Jul 2011 17:58:33 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Shan Hai <haishan.bai@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, paulus@...ba.org,
	tglx@...utronix.de, walken@...gle.com, dhowells@...hat.com,
	cmetcalf@...era.com, tony.luck@...el.com,
	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org
Subject: Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW
 tracking of dirty & young

On Tue, 2011-07-26 at 23:50 -0700, Mike Frysinger wrote:
> On Mon, Jul 18, 2011 at 21:29, Benjamin Herrenschmidt wrote:
> > The futex code currently attempts to write to user memory within
> > a pagefault disabled section, and if that fails, tries to fix it
> > up using get_user_pages().
> >
> > This doesn't work on archs where the dirty and young bits are
> > maintained by software, since they will gate access permission
> > in the TLB, and will not be updated by gup().
> >
> > In addition, there's an expectation on some archs that a
> > spurious write fault triggers a local TLB flush, and that is
> > missing from the picture as well.
> >
> > I decided that adding those "features" to gup() would be too much
> > for this already too complex function, and instead added a new
> > simpler fixup_user_fault() which is essentially a wrapper around
> > handle_mm_fault() which the futex code can call.
> 
> unfortunately, this breaks all nommu ports.  you added
> fixup_user_fault() to mm/memory.c only which is not used by nommu

Argh. Andrew, do you want to send a fix ? I won't be able to do that
tonight, I have to go.

What should nommu do anyways ? it's not like there's much it can do
right ? It should never even hit the fault path to start with ...

Cheers,
Ben.


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