[PATCH] powerpc/64s: Fix THP PMD collapse serialisation

[Nicholas Piggin]

Aneesh Kumar K.V's on June 7, 2019 1:23 am:
> Nicholas Piggin <npiggin at gmail.com> writes:
> 
>> Commit 1b2443a547f9 ("powerpc/book3s64: Avoid multiple endian conversion
>> in pte helpers") changed the actual bitwise tests in pte_access_permitted
>> by using pte_write() and pte_present() helpers rather than raw bitwise
>> testing _PAGE_WRITE and _PAGE_PRESENT bits.
>>
>> The pte_present change now returns true for ptes which are !_PAGE_PRESENT
>> and _PAGE_INVALID, which is the combination used by pmdp_invalidate to
>> synchronize access from lock-free lookups. pte_access_permitted is used by
>> pmd_access_permitted, so allowing GUP lock free access to proceed with
>> such PTEs breaks this synchronisation.
>>
>> This bug has been observed on HPT host, with random crashes and corruption
>> in guests, usually together with bad PMD messages in the host.
>>
>> Fix this by adding an explicit check in pmd_access_permitted, and
>> documenting the condition explicitly.
>>
>> The pte_write() change should be okay, and would prevent GUP from falling
>> back to the slow path when encountering savedwrite ptes, which matches
>> what x86 (that does not implement savedwrite) does.
>>
> 
> I guess we are doing the find_linux_pte change in another patch.
> 
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>

Sorry, just got delayed with re-testing. Thanks for the feedback on it
I send new patches.

Two patches yes because they fix issues introduced in different
commits so it should make backports easier.

Thanks,
Nick