Re: RFC: Non Power of 2 memory allocator

[Hugh Dickins <hugh@veritas.com>]

On Sat, 25 Mar 2006, Andi Kleen wrote:
> Robin Getz <rgetz@blackfin.uclinux.org> writes:
> 
> > The  buddy system allocates things in power of 2 pages sizes (4k, 8k,
> > 16k, 32k, 64k, 128k, 256k, 512k, 1024k), which works fine on most
> > systems, but an embedded system, which is running without a MMU (
> > Memory Management Unit) - RAM is precious, and when you only need
> > 129k for an application, you don't want to allocate a power of 2,
> > which gives you 256k -  an extra 127k, which can't be used by
> > anything else.
> 
> In 2.4 I solved this problem at some point by just returning
> the excess pages to the buddy allocator. There was even
> a nice function to do this (alloc_exact)
> 
> That won't work for slab, but does for __get_free_pages() which
> is better for large allocations anyways. slab imho doesn't make
> sense for allocation anywhere bigger PAGE_SIZE/2. At some
> point in 2.6 there was trouble with "compound pages" but I think
> that has been resolved. 
> 
> Just implementing alloc_exact again would be the simplest solution
> for your problem.

Nick has put a split_page function into the 2.6.16-git mm/page_alloc.c,
which I believe is supposed to be a helper in this kind of operation.
You'd best take a look at where and how it's used.  Perhaps Andi's
alloc_exact should be reimplemented in terms of it.

Hugh