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Subject: Re: [PATCH v6 2/8] mm,memory_hotplug: Relax fully spanned sections
 check
To:     Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20210406111115.8953-1-osalvador@suse.de>
 <20210406111115.8953-3-osalvador@suse.de>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <b915143d-a8df-ddfc-94a8-7578fdd5f7bc@redhat.com>
Date:   Tue, 6 Apr 2021 17:32:43 +0200
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On 06.04.21 13:11, 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.
> 
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
>   mm/memory_hotplug.c | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0cdbbfbc5757..5fe3e3942b19 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -838,9 +838,13 @@ 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.
> +	 */
>   	if (WARN_ON_ONCE(!nr_pages ||
> -			 !IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION)))
> +			 !IS_ALIGNED(pfn | nr_pages, pageblock_nr_pages)))
>   		return -EINVAL;
>   
>   	mem_hotplug_begin();
> @@ -1573,9 +1577,13 @@ 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 | nr_pages, pageblock_nr_pages)))
>   		return -EINVAL;
>   
>   	mem_hotplug_begin();
> 

I'd only relax start_pfn. That way the function is pretty much 
impossible to abuse for sub-section onlining/offlining.

if (WARN_ON_ONCE(!nr_pages ||
		 !IS_ALIGNED(start_pfn, pageblock_nr_pages))
		 !IS_ALIGNED(start_pfn + nr_pages, PAGES_PER_SECTION)))

-- 
Thanks,

David / dhildenb