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

[Li Yang]

On Fri, Nov 20, 2009 at 5:03 PM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
> On Fri, 2009-11-20 at 11:00 +0800, Li Yang-R58472 wrote:
>> Because there is no way to set mapped memory as cacheable if the
>> memory
>> is not managed by Linux kernel.  While, it's not rare in real system
>> to
>> allocate some dedicated memory to a certain application which is not
>> managed by kernel and then mmap'ed the memory to the application.  The
>> memory should be cacheable but we can't map it to be cacheable due to
>> this intelligent setting.  And it is a big hit to the performance.
>> Moreover, the standard O_SYNC flag suggest that user has the control
>> over cacheablity, but actually we had not.
>
> 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.

- Leo