Re: Page allocator: Single Zone optimizations

[Andrew Morton <akpm@osdl.org>]

On Fri, 27 Oct 2006 19:12:16 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> On Fri, 27 Oct 2006, Andrew Morton wrote:
> 
> > On Fri, 27 Oct 2006 18:00:42 -0700 (PDT)
> > Christoph Lameter <clameter@sgi.com> wrote:
> > 
> > > But I cannot find any justification in my contexts to complete work on 
> > > this functionality because plainly all the hardware that I use does not 
> > > have problem laden DMA controllers and works just fine with a single 
> > > zone.
> > 
> > How about memory hot-unplug?
> 
> Cannot figure out how that relates to what I said above.

We need some way of preventing unreclaimable kernel memory allocations from
using certain physical pages.  That means zones.

> Memory hot unplug 
> seems to have been dropped in favor of baloons.

Has it?  I don't recall seeing a vague proposal, let alone an implementation?

> > The only feasible way we're going to implement that is to support it on
> > user allocations only.  IOW: for all those allocations which were performed
> > with __GFP_HIGHMEM.
> 
> The alloc_page_range() functionality was intended for device drivers and 
> other ZONE_DMA users. I am not sure what the point is of user space 
> having the ability to allocate memory in specific physical memory areas. 

Userspace allocations are reclaimable: pagecache, anonymous memory.  These
happen to be allocated with __GFP_HIGHMEM set.

So right now __GFP_HIGHMEM is an excellent hint telling the page allocator
that it is safe to satisfy this request from removeable memory.


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