[RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young
benh at kernel.crashing.org
Wed Jul 27 17:58:33 EST 2011
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 ...
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