[PATCH 0/3] Simplify set_huge_pte_at, pte_same for LPAE

[Steve Capper]

Hello,
The following patch series is my attempt at fixing a rather nasty bug
which became visible in 3.12-rc1 when running the libhugetlbfs test
suite. (This problem only just came to my attention yesterday).

For LPAE, set_huge_pte_at calls set_pte_at which then calls
set_pte_ext, which in turn is wired up to call cpu_v7_set_pte_ext,
which is defined in proc-v7-3level.S.

For huge pages, given newprot a pgprot_t value for a shared writable
VMA, and ptep a pointer to a pte belonging to this VMA; the following
behaviour is assumed by core code:
   hugetlb_change_protection(vma, address, end, newprot);
   ...

   huge_pte_write(huge_ptep_get(ptep)); /* should be true! */

Unfortunately, cpu_v7_set_pte_ext will change the bit layout of the
resultant pte, and will set the read only bit if the dirty bit is not
also enabled.

If one were to allocate a read only shared huge page, then fault it in,
and then mprotect it to be writeable. A subsequent write to that huge
page will result in a spurious call to hugetlb_cow, which causes
corruption. This call is optimised away prior to:
 37a2140 mm, hugetlb: do not use a page in page cache for cow
         optimization

If one runs the libhugetlbfs test suite on v3.12-rc1 upwards, then the
mprotect test will cause the afformentioned corruption and before the
set of tests completes, the system will be left in an unresponsive
state. (calls to fork fail with -ENOMEM).

This was an absolute pig to debug and, as this is the second time I've
ran into issues caused by ptes being modified in transit, I've opted to
re-implement set_huge_pte_at such that it just dereferences the pte.
(in a similar manner as arm64). This has also allowed me to revert the
pte_same logic change (that removed the NG bit from comparison), by
also setting the NG bit for all new huge ptes.

These patches are against 3.12, and I have tested this series on an
Arndale board with LPAE running libhugetlbfs.

I would really value any comments/critique/flames on this series.
Especially as I've ommitted the DCCMVAC at the end of set_huge_pte_at
as I couldn't see why it was needed, please yell at me if it is needed!
:-)

Cheers,
-- 
Steve

Steve Capper (3):
  ARM: mm: Rewire LPAE set_huge_pte_at
  ARM: mm: Make LPAE huge page ptes NG by default
  Revert "ARM: mm: correct pte_same behaviour for LPAE."

 arch/arm/include/asm/hugetlb-3level.h |  7 ++++++-
 arch/arm/include/asm/pgtable-3level.h | 19 +------------------
 2 files changed, 7 insertions(+), 19 deletions(-)

-- 
1.8.1.4