linux-kernel - Re: [PATCH v3 5/7] mm/sparse.c: add note about only VMEMMAP supporting sub-section support

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Message-ID: <20200310144616.GM8447@dhcp22.suse.cz>
Date:   Tue, 10 Mar 2020 15:46:16 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, david@...hat.com,
        richardw.yang@...ux.intel.com, dan.j.williams@...el.com,
        osalvador@...e.de, rppt@...ux.ibm.com
Subject: Re: [PATCH v3 5/7] mm/sparse.c: add note about only VMEMMAP
 supporting sub-section support

On Sat 07-03-20 16:42:27, 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@...hat.com>

Acked-by: Michal Hocko <mhocko@...e.com>

I have glanced through other patches and they seem sane but I do not
have time to go deeper to give an ack. I like this one though because it
really makes the intention clearer.

> ---
>  mm/sparse.c | 37 ++++++++++++++++++++-----------------
>  1 file changed, 20 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 2142045ab5c5..0fbd79c4ad81 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -772,6 +772,22 @@ 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)
>  {
> @@ -784,23 +800,6 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  		return;
>  
>  	empty = is_subsection_map_empty(ms);
> -	/*
> -	 * 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
> -	 */
>  	if (empty) {
>  		unsigned long section_nr = pfn_to_section_nr(pfn);
>  
> @@ -907,6 +906,10 @@ 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.
> -- 
> 2.17.2
> 

-- 
Michal Hocko
SUSE Labs