Re: [PATCH] sparse: Track the boundaries of memory sections for accurate checks

[Michal Hocko]

On Sat 18-06-16 12:11:19, KarimAllah Ahmed wrote:
> When sparse memory model is used an array of memory sections is created to
> track each block of contiguous physical pages. Each element of this array
> contains PAGES_PER_SECTION pages. During the creation of this array the actual
> boundaries of the memory block is lost, so the whole block is either considered
> as present or not.
> 
> pfn_valid() in the sparse memory configuration checks which memory sections the
> pfn belongs to then checks whether it's present or not. This yields sub-optimal
> results when the available memory doesn't cover the whole memory section,
> because pfn_valid will return 'true' even for the unavailable pfns at the
> boundaries of the memory section.

Please be more verbose of _why_ the patch is needed. Why those
"sub-optimal results" matter?

> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Yaowei Bai <baiyaowei@xxxxxxxxxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Joe Perches <joe@xxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Anthony Liguori <aliguori@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> Signed-off-by: Jan H. Schönherr <jschoenh@xxxxxxxxx>
> ---
>  include/linux/mmzone.h | 22 ++++++++++++++++------
>  mm/sparse.c            | 37 ++++++++++++++++++++++++++++++++++++-
>  2 files changed, 52 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 02069c2..f76a0e1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1067,8 +1067,12 @@ struct mem_section {
>  	 * section. (see page_ext.h about this.)
>  	 */
>  	struct page_ext *page_ext;
> -	unsigned long pad;
> +	unsigned long pad[3];
>  #endif
> +
> +	unsigned long first_pfn;
> +	unsigned long last_pfn;
> +
>  	/*
>  	 * WARNING: mem_section must be a power-of-2 in size for the
>  	 * calculation and use of SECTION_ROOT_MASK to make sense.
> @@ -1140,23 +1144,29 @@ static inline int valid_section_nr(unsigned long nr)
>  
>  static inline struct mem_section *__pfn_to_section(unsigned long pfn)
>  {
> +	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> +		return NULL;
> +
>  	return __nr_to_section(pfn_to_section_nr(pfn));
>  }
>  
>  #ifndef CONFIG_HAVE_ARCH_PFN_VALID
>  static inline int pfn_valid(unsigned long pfn)
>  {
> -	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> +	struct mem_section *ms;
> +
> +	ms = __pfn_to_section(pfn);
> +
> +	if (ms && !(ms->first_pfn <= pfn && ms->last_pfn >= pfn))
>  		return 0;
> -	return valid_section(__nr_to_section(pfn_to_section_nr(pfn)));
> +
> +	return valid_section(ms);
>  }
>  #endif
>  
>  static inline int pfn_present(unsigned long pfn)
>  {
> -	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> -		return 0;
> -	return present_section(__nr_to_section(pfn_to_section_nr(pfn)));
> +	return present_section(__pfn_to_section(pfn));
>  }
>  
>  /*
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 5d0cf45..3c91837 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -166,24 +166,59 @@ void __meminit mminit_validate_memmodel_limits(unsigned long *start_pfn,
>  	}
>  }
>  
> +static int __init
> +overlaps(u64 start1, u64 end1, u64 start2, u64 end2)
> +{
> +	u64 start, end;
> +
> +	start = max(start1, start2);
> +	end = min(end1, end2);
> +	return start <= end;
> +}
> +
>  /* Record a memory area against a node. */
>  void __init memory_present(int nid, unsigned long start, unsigned long end)
>  {
> +	unsigned long first_pfn = start;
>  	unsigned long pfn;
>  
>  	start &= PAGE_SECTION_MASK;
>  	mminit_validate_memmodel_limits(&start, &end);
>  	for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION) {
>  		unsigned long section = pfn_to_section_nr(pfn);
> +		unsigned long last_pfn = min(pfn + PAGES_PER_SECTION, end) - 1;
>  		struct mem_section *ms;
>  
>  		sparse_index_init(section, nid);
>  		set_section_nid(section, nid);
>  
>  		ms = __nr_to_section(section);
> -		if (!ms->section_mem_map)
> +		if (!ms->section_mem_map) {
>  			ms->section_mem_map = sparse_encode_early_nid(nid) |
>  							SECTION_MARKED_PRESENT;
> +		} else {
> +			/* Merge the two regions */
> +			WARN_ON(sparse_early_nid(ms) != nid);
> +
> +			/*
> +			 * If they don't overlap there will be a hole in
> +			 * between where meta-data says it's valid even though
> +			 * it's not.
> +			 */
> +			if (!overlaps(first_pfn, last_pfn + 1,
> +				      ms->first_pfn, ms->last_pfn + 1))	{
> +				pr_info("Merging non-contiguous pfn ranges 0x%lx-0x%lx and 0x%lx-0x%lx\n",
> +					ms->first_pfn, ms->last_pfn,
> +					first_pfn, last_pfn);
> +			}
> +			first_pfn = min(first_pfn, ms->first_pfn);
> +			last_pfn = max(last_pfn, ms->last_pfn);
> +		}
> +
> +		ms->first_pfn = first_pfn;
> +		ms->last_pfn = last_pfn;
> +
> +		first_pfn = pfn + PAGES_PER_SECTION;
>  	}
>  }
>  
> -- 
> 2.8.2
> 

-- 
Michal Hocko
SUSE Labs