[RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem alignment

[Mike Rapoport]

On Wed, Jul 29, 2020 at 03:03:04PM +0200, David Hildenbrand wrote:
> On 29.07.20 15:00, Mike Rapoport wrote:
> > On Wed, Jul 29, 2020 at 11:35:20AM +0200, David Hildenbrand wrote:
> >>>  
> >>> There is still large gap with ARM64_64K_PAGES, though.
> >>>
> >>> As for SPARSEMEM without VMEMMAP, are there actual benefits to use it?
> >>
> >> I was asking myself the same question a while ago and didn't really find
> >> a compelling one.
> > 
> > Memory overhead for VMEMMAP is larger, especially for arm64 that knows
> > how to free empty parts of the memory map with "classic" SPARSEMEM.
> 
> You mean the hole punching within section memmap? (which is why their
> pfn_valid() implementation is special)

Yes, arm (both 32 and 64) do this. And for smaller systems with a few
memory banks this is very reasonable to trade slight (if any) slowdown
in pfn_valid() for several megs of memory.
 
> (I do wonder why that shouldn't work with VMEMMAP, or is it simply not
> implemented?)
 
It's not implemented. There was a patch [1] recently to implement this. 

[1] https://lore.kernel.org/lkml/20200721073203.107862-1-liwei213@huawei.com/

> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.