[RFC V2] ARM: mm: make UACCESS_WITH_MEMCPY huge page aware

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Sep 23, 2013 at 02:19:24PM +0100, Steve Capper wrote:
> On Tue, Sep 17, 2013 at 10:33:49AM +0100, Steve Capper wrote:
> > The memory pinning code in uaccess_with_memcpy.c does not check
> > for HugeTLB or THP pmds, and will enter an infinite loop should
> > a __copy_to_user or __clear_user occur against a huge page.
> > 
> > This patch adds detection code for huge pages to pin_page_for_write.
> > As this code can be executed in a fast path it refers to the actual
> > pmds rather than the vma. If a HugeTLB or THP is found (they have
> > the same pmd representation on ARM), the page table spinlock is
> > taken to prevent modification whilst the page is pinned.
> > 
> > On ARM, huge pages are only represented as pmds, thus no huge pud
> > checks are performed. (For huge puds one would lock the page table
> > in a similar manner as in the pmd case).
> > 
> > Two helper functions are introduced; pmd_thp_or_huge will check
> > whether or not a page is huge or transparent huge (which have the
> > same pmd layout on ARM), and pmd_hugewillfault will detect whether
> > or not a page fault will occur on write to the page.
> > 
> > Changes since first RFC:
> >    * The page mask is widened for hugepages to reduce the number
> >      of potential locks/unlocks.
> >      (A knobbled /dev/zero with its latency reduction chunks
> >       removed shows a 2x data rate boost with hugepages backing:
> >       dd if=/dev/zero of=/dev/null bs=10M count=1024 )
> > 
> > Signed-off-by: Steve Capper <steve.capper@linaro.org>
> > ---
> 
> A ping to see if anyone had any questions/comments/flames :-)?
> 
> Should it go into the patch system?

IIRC Nicolas wrote the original support, so he should probably be Cc'd with
this patch.