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.

Cheers,
Ben.