Rough notes from Kernel Consolidation meeting

[Catalin Marinas]

On Thu, 2010-09-23 at 16:15 +0100, Arnd Bergmann wrote:
> On Thursday 23 September 2010, Nicolas Pitre wrote:
> > >  This highmem topic comes from the fact that highmem will be needed in
> > >  the period of time between now and LPAE where we have boards with lots
> > >  of memory but we can't address it all without highmem (unless we want
> > >  to revisit the 3g/1g split, but I personally think not).
> >
> > Note that LPAE does require highmem to be useful.  The only way highmem
> > could be avoided is to move to a 64-bit architecture.
> 
> Right, I'd even say LPAE can only make things worse because people
> will stick even more memory into their systems, most of which then
> becomes highmem.

If you really need so much memory, it's more efficient to have LPAE
+highmem than a swap device. The problem is if the OS doesn't need so
much memory but it is available, Linux tries to allocate from highmem
first. What could help is a different zone fall back mechanism trying to
allocate from lowmem up to a certain threshold.

Another option would be to use the highmem for hosting a swap via some
form of ramdisk or slram/phram.

Yet another option is some dynamic memory hotplug based on the amount of
spare memory you've got.

> We might be able to use MMU features to implement a 4G/4G split, which
> lets us use 3GB physical RAM or more (depending on vmalloc and I/O sizes)
> without highmem, but can have an even higher cost by significantly
> slowing down uaccess.

It would be tricky to create temporary mappings for uaccess (and may
involve get_user_pages or some form of pinning the pages in memory).

-- 
Catalin