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

[steve.capper@linaro.org (Steve Capper)]

On Mon, Sep 23, 2013 at 02:21:51PM +0100, Russell King - ARM Linux wrote:
> 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.
> 

Thank you, I will resend the patch with Nicolas on CC.