[RFC] powerpc/mm: honor O_SYNC flag for memory map

[Li Yang]

On Sun, Nov 22, 2009 at 4:01 AM, Segher Boessenkool
<segher at kernel.crashing.org> wrote:
>>> You need to be a bit more careful tho. You must not allow RAM managed by
>>> the kernel to be mapped non-cachable.
>>
>> Even if the user explicitly sets the O_SYNC flag?  IMHO, it's a bug of
>> the application if it uses O_SYNC on main memory to be mmap'ed later.
>> And we don't need to cover up the bug.
>
> Is that "embedded thinking"?  Conflicts like this cause machine checks or
> checkstops on many PowerPC implementations, we do not normally allow such
> to be caused by userland.

So what you are saying is that if the kernel has mapped a physical
page as cacheable while user application is trying to map it as
non-cacheable, there will be machine checks and checkstops rather than
just performance drop?  This is new to me.  Could you elaborate a bit?
 Thanks.

- Leo