Re: [RFC PATCH] mm: set memory section offline when all its pages are offline.

[David Hildenbrand <david@redhat.com>]

On 08.10.19 16:36, lantianyu1986@gmail.com wrote:
> From: Tianyu Lan <Tianyu.Lan@microsoft.com>
> 
> If size of offline memory region passed to offline_pages() is
> not aligned with PAGES_PER_SECTION, memory section will be set
> to offline in the offline_mem_sections() with some pages of
> memory section online. Fix it, Update memory section status after
> marking offline pages as "reserved" in __offline_isolated_pages()
> and check all pages in memory are reserved or not before setting
> memory section offline.
> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
> ---
> This patch is to prepare for hot remove memory function in Hyper-V
> balloon driver. It requests to offline memory with random size.

I proposed roughly the same a while ago. See

https://lkml.org/lkml/2018/4/30/207

Memory onlining/offlining works in memory block granularity only.
Sub-sections, you have to emulate it on top, similar to how hyper-v
balloon handles it already. E.g., have a look how virtio-mem handles it
using alloc_contig_range/free_contig_range and PG_offline extensions.

https://lkml.org/lkml/2018/4/30/207

So a clear NACK from my side.


> ---
>  mm/page_alloc.c |  3 ++-
>  mm/sparse.c     | 10 ++++++++++
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index dbd0d5cbbcbb..cc02866924ae 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8540,7 +8540,6 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
>  	if (pfn == end_pfn)
>  		return offlined_pages;
>  
> -	offline_mem_sections(pfn, end_pfn);
>  	zone = page_zone(pfn_to_page(pfn));
>  	spin_lock_irqsave(&zone->lock, flags);
>  	pfn = start_pfn;
> @@ -8576,6 +8575,8 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
>  	}
>  	spin_unlock_irqrestore(&zone->lock, flags);
>  
> +	offline_mem_sections(pfn, end_pfn);
> +
>  	return offlined_pages;
>  }
>  #endif
> diff --git a/mm/sparse.c b/mm/sparse.c
> index fd13166949b5..eb5860487b84 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -571,6 +571,7 @@ void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>  void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>  {
>  	unsigned long pfn;
> +	int i;
>  
>  	for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
>  		unsigned long section_nr = pfn_to_section_nr(pfn);
> @@ -583,6 +584,15 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>  		if (WARN_ON(!valid_section_nr(section_nr)))
>  			continue;
>  
> +		/*
> +		 * Check whether all pages in the section are reserverd before
> +		 * setting setction offline.
> +		 */
> +		for (i = 0; i < PAGES_PER_SECTION; i++)
> +			if (!PageReserved(pfn_to_page(
> +			    SECTION_ALIGN_DOWN(pfn + i))))
> +				continue;
> +
>  		ms = __nr_to_section(section_nr);
>  		ms->section_mem_map &= ~SECTION_IS_ONLINE;
>  	}
> 


-- 

Thanks,

David / dhildenb