Re: [PATCH v6 01/12] mm/sparsemem: Introduce struct mem_section_usage

[Pavel Tatashin]

On 19-04-17 11:39:00, Dan Williams wrote:
> Towards enabling memory hotplug to track partial population of a
> section, introduce 'struct mem_section_usage'.
> 
> A pointer to a 'struct mem_section_usage' instance replaces the existing
> pointer to a 'pageblock_flags' bitmap. Effectively it adds one more
> 'unsigned long' beyond the 'pageblock_flags' (usemap) allocation to
> house a new 'map_active' bitmap.  The new bitmap enables the memory
> hot{plug,remove} implementation to act on incremental sub-divisions of a
> section.
> 
> The primary motivation for this functionality is to support platforms
> that mix "System RAM" and "Persistent Memory" within a single section,
> or multiple PMEM ranges with different mapping lifetimes within a single
> section. The section restriction for hotplug has caused an ongoing saga
> of hacks and bugs for devm_memremap_pages() users.
> 
> Beyond the fixups to teach existing paths how to retrieve the 'usemap'
> from a section, and updates to usemap allocation path, there are no
> expected behavior changes.
> 
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  include/linux/mmzone.h |   23 ++++++++++++--
>  mm/memory_hotplug.c    |   18 ++++++-----
>  mm/page_alloc.c        |    2 +
>  mm/sparse.c            |   81 ++++++++++++++++++++++++------------------------
>  4 files changed, 71 insertions(+), 53 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 70394cabaf4e..f0bbd85dc19a 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1160,6 +1160,19 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
>  #define SECTION_ALIGN_UP(pfn)	(((pfn) + PAGES_PER_SECTION - 1) & PAGE_SECTION_MASK)
>  #define SECTION_ALIGN_DOWN(pfn)	((pfn) & PAGE_SECTION_MASK)
>  
> +#define SECTION_ACTIVE_SIZE ((1UL << SECTION_SIZE_BITS) / BITS_PER_LONG)
> +#define SECTION_ACTIVE_MASK (~(SECTION_ACTIVE_SIZE - 1))
> +
> +struct mem_section_usage {
> +	/*
> +	 * SECTION_ACTIVE_SIZE portions of the section that are populated in
> +	 * the memmap
> +	 */
> +	unsigned long map_active;

I think this should be proportional to section_size / subsection_size.
For example, on intel section size = 128M, and subsection is 2M, so
64bits work nicely. But, on arm64 section size if 1G, so subsection is
16M.

On the other hand 16M is already much better than what we have: with 1G
section size and 2M pmem alignment we guaranteed to loose 1022M. And
with 16M subsection it is only 14M.