linux-kernel - Re: [PATCH v9 2/8] mm,memory_hotplug: Relax fully spanned sections check

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Date:   Tue, 20 Apr 2021 11:40:50 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 2/8] mm,memory_hotplug: Relax fully spanned sections
 check

On Fri 16-04-21 13:24:05, Oscar Salvador wrote:
> When using self-hosted vmemmap pages, the number of pages passed to
> {online,offline}_pages might not fully span sections, but they always
> fully span pageblocks.
> Relax the check account for that case.

It would be good to call those out explicitly.  It would be also
great to explain why pageblock_nr_pages is an actual constrain. There
shouldn't be any real reason for that except for "we want online_pages
to operate on whole memblocks and memmap_on_memory will poke
pageblock_nr_pages aligned holes in the beginning which is a special
case we want to allow."

> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> Reviewed-by: David Hildenbrand <david@...hat.com>

With the changelog extended and the comment clarification (se below)
feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/memory_hotplug.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0cdbbfbc5757..25e59d5dc13c 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -838,9 +838,14 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
>  	int ret;
>  	struct memory_notify arg;
>  
> -	/* We can only online full sections (e.g., SECTION_IS_ONLINE) */
> +	/* We can only offline full sections (e.g., SECTION_IS_ONLINE).
> +	 * However, when using e.g: memmap_on_memory, some pages are initialized
> +	 * prior to calling in here. The remaining amount of pages must be
> +	 * pageblock aligned.


I would rephrase (and also note that multi line comment usually have a
leading line without any content - not that I care much though).

	/*
	 * {on,off}lining is constrained to full memory sections (or
	 * more precisly to memory blocks from the user space POV).
	 * memmap_on_memory is an exception because it reserves initial
	 * part of the physical memory space for vmemmaps. That space is
	 * pageblock aligned.
> +	 */

Same comment would apply to oofline_pages.

>  	if (WARN_ON_ONCE(!nr_pages ||
> -			 !IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION)))
> +			 !IS_ALIGNED(pfn, pageblock_nr_pages) ||
> +			 !IS_ALIGNED(pfn + nr_pages, PAGES_PER_SECTION)))
>  		return -EINVAL;
>  
>  	mem_hotplug_begin();
> @@ -1573,9 +1578,14 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  	int ret, node;
>  	char *reason;
>  
> -	/* We can only offline full sections (e.g., SECTION_IS_ONLINE) */
> +	/* We can only offline full sections (e.g., SECTION_IS_ONLINE).
> +	 * However, when using e.g: memmap_on_memory, some pages are initialized
> +	 * prior to calling in here. The remaining amount of pages must be
> +	 * pageblock aligned.
> +	 */
>  	if (WARN_ON_ONCE(!nr_pages ||
> -			 !IS_ALIGNED(start_pfn | nr_pages, PAGES_PER_SECTION)))
> +			 !IS_ALIGNED(start_pfn, pageblock_nr_pages) ||
> +			 !IS_ALIGNED(start_pfn + nr_pages, PAGES_PER_SECTION)))
>  		return -EINVAL;
>  
>  	mem_hotplug_begin();
> -- 
> 2.16.3

-- 
Michal Hocko
SUSE Labs