Re: [PATCH v4 4/5] mm/sparse.c: add note about only VMEMMAP supporting sub-section hotplug

[David Hildenbrand <david@redhat.com>]

On 12.03.20 13:44, Baoquan He wrote:
> And tell check_pfn_span() gating the porper alignment and size of
> hot added memory region.
> 
> And also move the code comments from inside section_deactivate()
> to being above it. The code comments are reasonable for the whole
> function, and the moving makes code cleaner.
> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/sparse.c | 38 +++++++++++++++++++++-----------------
>  1 file changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 117fe4554c38..f02a524e17d1 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -771,6 +771,22 @@ static bool is_subsection_map_empty(struct mem_section *ms)
>  }
>  #endif
>  
> +/*
> + * To deactivate a memory region, there are 3 cases to handle across
> + * two configurations (SPARSEMEM_VMEMMAP={y,n}):
> + *
> + * 1. deactivation of a partial hot-added section (only possible in
> + *    the SPARSEMEM_VMEMMAP=y case).
> + *      a) section was present at memory init.
> + *      b) section was hot-added post memory init.
> + * 2. deactivation of a complete hot-added section.
> + * 3. deactivation of a complete section from memory init.
> + *
> + * For 1, when subsection_map does not empty we will not be freeing the
> + * usage map, but still need to free the vmemmap range.
> + *
> + * For 2 and 3, the SPARSEMEM_VMEMMAP={y,n} cases are unified
> + */
>  static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  		struct vmem_altmap *altmap)
>  {
> @@ -781,23 +797,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  
>  	if (clear_subsection_map(pfn, nr_pages))
>  		return;
> -	/*
> -	 * There are 3 cases to handle across two configurations
> -	 * (SPARSEMEM_VMEMMAP={y,n}):
> -	 *
> -	 * 1/ deactivation of a partial hot-added section (only possible
> -	 * in the SPARSEMEM_VMEMMAP=y case).
> -	 *    a/ section was present at memory init
> -	 *    b/ section was hot-added post memory init
> -	 * 2/ deactivation of a complete hot-added section
> -	 * 3/ deactivation of a complete section from memory init
> -	 *
> -	 * For 1/, when subsection_map does not empty we will not be
> -	 * freeing the usage map, but still need to free the vmemmap
> -	 * range.
> -	 *
> -	 * For 2/ and 3/ the SPARSEMEM_VMEMMAP={y,n} cases are unified
> -	 */
> +
>  	empty = is_subsection_map_empty(ms);
>  	if (empty) {
>  		unsigned long section_nr = pfn_to_section_nr(pfn);
> @@ -905,6 +905,10 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn,
>   *
>   * This is only intended for hotplug.
>   *
> + * Note that only VMEMMAP supports sub-section aligned hotplug,
> + * the proper alignment and size are gated by check_pfn_span().
> + *
> + *
>   * Return:
>   * * 0		- On success.
>   * * -EEXIST	- Section has been present.
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb