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

[Mike Frysinger]

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