Re: [LSF/MM/BFP TOPIC] Deprecate SPARSEMEM and have only SPARSEMEM_VMEMMAP

[Mike Rapoport <rppt@kernel.org>]

On Fri, May 10, 2024 at 11:03:14AM +0200, Oscar Salvador wrote:
> Hi all,
> 
> I would like to discuss the following topic in the LSFMM.
> 
> We have SPARSEMEM memory model and SPARSEMEM_VMEMMAP, where the only difference
> between the two of them is that the latter allocates a virtual chunk to represent
> the memmap array, which speeds operations like
> page_to_pfn/pfn_to_page/nth_page/folio_page_idx.
 
...
 
> I did some research on which arches use CONFIG_SPARSE_MEMMAP/VMEMMAP or
> none(using flatmem?).

FLATMEM it is.
                    
>  SPARSE_MEMMAP       SPARSE_VMEMMAP
>  arc
>  arm              X
>  arm64            X                     X
>  csky
>  hexagon 
>  loongarch        X                     X
>  m68k
>  microblaze
>  mips             X
>  nios2
>  openrisc
>  parisc
>  powerpc          X                     X
>  riscv            X                     X
>  s390             X                     X
>  sh               X
>  sparc            X                     X
>  um
>  x86              X                     X
>  xtensa
> 
> arm, mips, parisc and sh operate with SPARSE_MEMMAP but are lacking code for
> SPARSE_VMEMMAP.

arm can use "sparse flatmem" in the sense that it can free unused parts of
the memory map so FLATMEM does not have memory overhead with sparsely
located memory banks. So possible we can just drop SPARSEMEM on arm.

But I agree with Matthew that for this discussion we'd need arch
maintainers to participate.

> Thanks
> 
> -- 
> Oscar Salvador
> SUSE Labs
> 

-- 
Sincerely yours,
Mike.