On Fri, Nov 20, 2009 at 5:03 PM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Fri, 2009-11-20 at 11:00 +0800, Li Yang-R58472 wrote:

quoted

Because there is no way to set mapped memory as cacheable if the
memory
is not managed by Linux kernel. =C2=A0While, it's not rare in real syste=

m

quoted

to
allocate some dedicated memory to a certain application which is not
managed by kernel and then mmap'ed the memory to the application. =C2=A0=

The

quoted

memory should be cacheable but we can't map it to be cacheable due to
this intelligent setting. =C2=A0And 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