On Tue, Jul 13, 2010 at 10:46:12AM +0100, Russell King - ARM Linux wrote: > On Tue, Jul 13, 2010 at 10:37:00AM +0100, Mel Gorman wrote: > > I prefer Kamezawa's suggestion of mapping on a ZERO_PAGE-like page full > > of PageReserved struct pages because it would have better performance > > and be more in line with maintaining the assumptions of the memory > > model. If we go in this direction, I would strongly prefer it was an > > ARM-only thing. > > As I've said, this is not possible without doing some serious page > manipulation. > Yep, x86 used to do something like this for discontig. It wasn't pretty. > Plus the pages that where there become unusable as they don't correspond > with a PFN or obey phys_to_virt(). So there's absolutely no point to > this. > > Now, why do we free the holes in the mem_map - because these holes can > be extremely large. Every 512K of hole equates to one page of mem_map > array. Sure, the holes might be large but at least they are contiguous. Is there ever a case where you have 512K_Valid 512K_Hole 512K_Valid 512K_Hole or is it typically 512K_hole 512K_hole ...... 512K_Valid 512K_Valid etc If holes are typically contiguos, memmap is not allocated in the first place and the savings from punching holes in memmap in the latter configuration are minimal. I recognise if you have a 2M section with a hole in it, you are potentially wasting 3 pages on unused memmap. If this is really a problem, Minchan's modification to sparsemem to increase the size of mem_section on CONFIG_ARCH_HAS_HOLES_MEMORYMODEL is a messy option. I say messy because it only works if the hole is on either end of the section and it's adding quirks to the memory model. > Balance that against memory placed at 0xc0000000 physical on > some platforms, and with PHYSMEM_BITS at 32 and SECTION_SIZE_BITS at > 19 - well, you do the maths. The result is certainly not pretty. > -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>