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avagin 
arm: Workaround shmat() page coloring alignment

Please, see incode comment about the issue.
There was an attempt to fix this feature in kernel:
  http://www.spinics.net/lists/arm-kernel/msg258870.html

I'll send a patch for kernel with a correct justification.
Still it's worth to workaround this on older kernels.
mremap() after shmat() makes it possible to C/R shmem between ARMv7 CPUs.
Without it C/R will fail even on the same platform.

That is possible that this workaround changes *restore failure* to
*corruption* in shared memory after restore.
Still, I think it's worth to be applied by the following reasons:
1. All ARMv7 CPUs do not have VIPT aliasing data cache.
   And ARMv6 CPUs may have any, but they are not popular because of UMP.
2. SysV IPC shmem should be writable and SHMLBA unaligned
   (if it's only readable, then it's OK).
3. For the data corruption we need to migrate from non-VIPT cached CPUs
   to VIPT aliasing.
4. As this is shmem migration between platforms - quite likely we have
   it`s copy (in case of whole IPC namespace dump - in images).
5. C/R before on the same CPU may fail (quite likely, more than 50% of
   zdtm/transition/ipc failed while I've test it).

So, I think it's very unlikely that someone uses CRIU to migrate
from ARMv7 to ARMv6 unaligned writable shmem, but if he is - here is
a BIG WARNING on restore.

And after all: we've restored application as it was on checkpoint,
we can't reliably tell that shmem needs align on the target platform,
but we warned on restoring.

I wanted to test the cpuid for VIPT caching firstly, but it's in cp15
coprocessor, which means it's unavailable from userspace. Only one
reliable way to tell is to check couple of first boot lines in dmesg:
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instruction cache

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

18179 of 27192 relevant lines covered (66.85%)

58020.79 hits per line

Jobs
ID Job ID Ran Files Coverage
1 923.1 (TR_ARCH=local GCOV=1) 211
66.85
 Travis Job 923.1
Source Files on build 923
Coverage ∆ File Lines Relevant Covered Missed Hits/Line