This manifested as grep failing psuedo-randomly:

-------------->8---------------------
[ARCLinux]$ ip address show lo | grep inet
[ARCLinux]$ ip address show lo | grep inet
[ARCLinux]$ ip address show lo | grep inet
[ARCLinux]$
[ARCLinux]$ ip address show lo | grep inet
    inet 127.0.0.1/8 scope host lo
-------------->8---------------------

ARC700 MMU provides fully orthogonal permission bits per page:
Ur, Uw, Ux, Kr, Kw, Kx

The user mode page permission templates used to have all Kernel mode
access bits enabled.
This caused a tricky race condition observed with uClibc buffered file
read and UNIX pipes.

1. Read access to an anon mapped page in libc .bss: write-protected
   zero_page mapped: TLB Entry installed with Ur + K[rwx]

2. grep calls libc:getc() -> buffered read layer calls read(2) with the
   internal read buffer in same .bss page.
   The read() call is on STDIN which has been redirected to a pipe.
   read(2) => sys_read() => pipe_read() => copy_to_user()

3. Since page has Kernel-write permission (despite being user-mode
   write-protected), copy_to_user() suceeds w/o taking a MMU TLB-Miss
   Exception (page-fault for ARC). core-MM is unaware that kernel
   erroneously wrote to the reserved read-only zero-page (BUG #1)

4. Control returns to userspace which now does a write to same .bss page
   Since Linux MM is not aware that page has been modified by kernel, it
   simply reassigns a new writable zero-init page to mapping, loosing the
   prior write by kernel - effectively zero'ing out the libc read buffer
   under the hood - hence grep doesn't see right data (BUG #2)

The fix is to make all kernel-mode access permissions mirror the
user-mode ones. Note that the kernel still has full access to pages,
when accessed directly (w/o MMU) - this fix ensures that kernel-mode
access in copy_to_from() path uses the same faulting access model as for
pure user accesses to keep MM fully aware of page state.

The issue is peudo-random because it only shows up if the TLB entry
installed in #1 is present at the time of #3. If it is evicted out, due
to TLB pressure or some-such, then copy_to_user() does take a TLB Miss
Exception, with a routine write-to-anon COW processing installing a
fresh page for kernel writes and also usable as it is in userspace.

Further the issue was dormant for so long as it depends on where the
libc internal read buffer (in .bss) is mapped at runtime.
If it happens to reside in file-backed data mapping of libc (in the
page-aligned slack space trailing the file backed data), loader zero
padding the slack space, does the early cow page replacement, setting
things up at the very beginning itself.

With gcc 4.8 based builds, the libc buffer got pushed out to a real
anon mapping which triggers the issue.
Reported-by: NAnton Kolesov <akolesov@synopsys.com>
Cc: <stable@vger.kernel.org> # 3.9
Signed-off-by: NVineet Gupta <vgupta@synopsys.com>

This is the meat of the series which prevents any dcache alias creation
by always keeping the U and K mapping of a page congruent.
If a mapping already exists, and other tries to access the page, prev
one is flushed to physical page (wback+inv)

Essentially flush_dcache_page()/copy_user_highpage() create K-mapping
of a page, but try to defer flushing, unless U-mapping exist.
When page is actually mapped to userspace, update_mmu_cache() flushes
the K-mapping (in certain cases this can be optimised out)

Additonally flush_cache_mm(), flush_cache_range(), flush_cache_page()
handle the puring of stale userspace mappings on exit/munmap...

flush_anon_page() handles the existing U-mapping for anon page before
kernel reads it via the GUP path.

Note that while not complete, this is enough to boot a simple
dynamically linked Busybox based rootfs
Signed-off-by: NVineet Gupta <vgupta@synopsys.com>

munmap ends up calling tlb_flush() which for ARC was flushing the entire
TLB unconditionally (by moving the MMU to a new ASID)

do_munmap
  unmap_region
    unmap_vmas
      unmap_single_vma
         unmap_page_range
            tlb_start_vma
            zap_pud_range
            tlb_end_vma()
  tlb_finish_mmu
    tlb_flush()  ---> unconditional flush_tlb_mm()

So even a single page munmap, a frequent operation when uClibc dynamic
linker (ldso) is loading the dependent shared libraries, would move the
the ASID multiple times - needlessly invalidating the pre-faulted TLB
entries (and increasing the rate of ASID wraparound + full TLB flush).

This is now optimised to only be called if tlb->full_mm (which means
for exit/execve) cases only. And for those cases, flush_tlb_mm() is
already optimised to be a no-op for mm->mm_users == 0.

So essentially there are no mmore full mm flushes - except for fork which
anyhow needs it for properly COW'ing parent address space.

munmap now needs to do TLB range flush, which is implemented with
tlb_end_vma()

Results
-------
1. ASID now consistenly moves by 4 during a simple ls (as opposed to 5 or
   7 before).

2. LMBench microbenchmark also shows improvements

Basic system parameters
------------------------------------------------------------------------------
Host                 OS Description              Mhz  tlb  cache  mem scal
                                                     pages line   par load
                                                           bytes
--------- ------------- ----------------------- ---- ----- ----- ------ ----
3.9-rc5-0 Linux 3.9.0-r 3.9-rc5-0404-gcc-4.4-ba   80     8    64 1.1000 1
3.9-rc5-0 Linux 3.9.0-r 3.9-rc5-0405-avoid-full   80     8    64 1.1200 1

Processor, Processes - times in microseconds - smaller is better
------------------------------------------------------------------------------
Host                 OS  Mhz null null      open slct sig  sig  fork exec sh
                             call  I/O stat clos TCP  inst hndl proc proc proc
--------- ------------- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
3.9-rc5-0 Linux 3.9.0-r   80 4.81 8.69 68.6 118. 239. 8.53 31.6 4839 13.K 34.K
3.9-rc5-0 Linux 3.9.0-r   80 4.46 8.36 53.8 91.3 223. 8.12 24.2 4725 13.K 33.K

File & VM system latencies in microseconds - smaller is better
-------------------------------------------------------------------------------
Host                 OS   0K File      10K File     Mmap    Prot   Page 100fd
                        Create Delete Create Delete Latency Fault  Fault selct
--------- ------------- ------ ------ ------ ------ ------- ----- ------- -----
3.9-rc5-0 Linux 3.9.0-r  314.7  223.2 1054.9  390.2  3615.0 1.590 20.1 126.6
3.9-rc5-0 Linux 3.9.0-r  265.8  183.8 1014.2  314.1  3193.0 6.910 18.8 110.4
Signed-off-by: NVineet Gupta <vgupta@synopsys.com>

Signed-off-by: NVineet Gupta <vgupta@synopsys.com>

* MMU I-TLB / D-TLB Miss Exceptions
  - Fast Path TLB Refill Handler
  - slowpath TLB creation via do_page_fault() -> update_mmu_cache()
* Duplicate PD Exception Handler
Signed-off-by: NVineet Gupta <vgupta@synopsys.com>