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

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Message-ID: <CAMjpGUdxpaYBFfKjBEPFOJohnKoXfagLUHvhrst+NembeabxcA@mail.gmail.com>
Date:	Tue, 26 Jul 2011 23:50:46 -0700
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
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 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
logic.
-mike
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