Re: [PATCH v5 1/1] mm: refactor initialization of struct page for holes in memory layout

[David Hildenbrand <david@redhat.com>]

On 08.02.21 12:08, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> There could be struct pages that are not backed by actual physical memory.
> This can happen when the actual memory bank is not a multiple of
> SECTION_SIZE or when an architecture does not register memory holes
> reserved by the firmware as memblock.memory.
> 
> Such pages are currently initialized using init_unavailable_mem() function
> that iterates through PFNs in holes in memblock.memory and if there is a
> struct page corresponding to a PFN, the fields of this page are set to
> default values and it is marked as Reserved.
> 
> init_unavailable_mem() does not take into account zone and node the page
> belongs to and sets both zone and node links in struct page to zero.
> 
> On a system that has firmware reserved holes in a zone above ZONE_DMA, for
> instance in a configuration below:
> 
> 	# grep -A1 E820 /proc/iomem
> 	7a17b000-7a216fff : Unknown E820 type
> 	7a217000-7bffffff : System RAM
> 
> unset zone link in struct page will trigger
> 
> 	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
> 
> because there are pages in both ZONE_DMA32 and ZONE_DMA (unset zone link
> in struct page) in the same pageblock.
> 
> Moreover, it is possible that the lowest node and zone start is not aligned
> to the section boundarie, for example on x86:
> 
> [    0.078898] Zone ranges:
> [    0.078899]   DMA      [mem 0x0000000000001000-0x0000000000ffffff]
> ...
> [    0.078910] Early memory node ranges
> [    0.078912]   node   0: [mem 0x0000000000001000-0x000000000009cfff]
> [    0.078913]   node   0: [mem 0x0000000000100000-0x000000003fffffff]
> 
> and thus with SPARSEMEM memory model the beginning of the memory map will
> have struct pages that are not spanned by any node and zone.
> 
> Update detection of node boundaries in get_pfn_range_for_nid() so that the
> node range will be expanded to cover memory map section. Since zone spans
> are derived from the node span, there always will be a zone that covers the
> part of the memory map with unavailable pages.
> 
> Interleave initialization of the unavailable pages with the normal
> initialization of memory map, so that zone and node information will be
> properly set on struct pages that are not backed by the actual memory.
> 
> Fixes: 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions rather
> that check each PFN")
> Reported-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Qian Cai <cai@lca.pw>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> ---
>   mm/page_alloc.c | 160 +++++++++++++++++++++++-------------------------
>   1 file changed, 75 insertions(+), 85 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6446778cbc6b..1c3f7521028f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6257,22 +6257,84 @@ static void __meminit zone_init_free_lists(struct zone *zone)
>   	}
>   }
>   
> +#if !defined(CONFIG_FLAT_NODE_MEM_MAP)
> +/*
> + * Only struct pages that correspond to ranges defined by memblock.memory
> + * are zeroed and initialized by going through __init_single_page() during
> + * memmap_init_zone().
> + *
> + * But, there could be struct pages that correspond to holes in
> + * memblock.memory. This can happen because of the following reasons:
> + * - phyiscal memory bank size is not necessarily the exact multiple of the
> + *   arbitrary section size
> + * - early reserved memory may not be listed in memblock.memory
> + * - memory layouts defined with memmap= kernel parameter may not align
> + *   nicely with memmap sections
> + *
> + * Explicitly initialize those struct pages so that:
> + * - PG_Reserved is set
> + * - zone and node links point to zone and node that span the page
> + */
> +static u64 __meminit init_unavailable_range(unsigned long spfn,
> +					    unsigned long epfn,
> +					    int zone, int node)
> +{
> +	unsigned long pfn;
> +	u64 pgcnt = 0;
> +
> +	for (pfn = spfn; pfn < epfn; pfn++) {
> +		if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages))) {
> +			pfn = ALIGN_DOWN(pfn, pageblock_nr_pages)
> +				+ pageblock_nr_pages - 1;
> +			continue;
> +		}
> +		__init_single_page(pfn_to_page(pfn), pfn, zone, node);
> +		__SetPageReserved(pfn_to_page(pfn));
> +		pgcnt++;
> +	}
> +
> +	return pgcnt;
> +}
> +#else
> +static inline u64 init_unavailable_range(unsigned long spfn, unsigned long epfn,
> +					 int zone, int node)
> +{
> +	return 0;
> +}
> +#endif
> +
>   void __meminit __weak memmap_init_zone(struct zone *zone)
>   {
>   	unsigned long zone_start_pfn = zone->zone_start_pfn;
>   	unsigned long zone_end_pfn = zone_start_pfn + zone->spanned_pages;
>   	int i, nid = zone_to_nid(zone), zone_id = zone_idx(zone);
>   	unsigned long start_pfn, end_pfn;
> +	unsigned long hole_pfn = 0;
> +	u64 pgcnt = 0;
>   
>   	for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, NULL) {
>   		start_pfn = clamp(start_pfn, zone_start_pfn, zone_end_pfn);
>   		end_pfn = clamp(end_pfn, zone_start_pfn, zone_end_pfn);
> +		hole_pfn = clamp(hole_pfn, zone_start_pfn, zone_end_pfn);
>   
>   		if (end_pfn > start_pfn)
>   			memmap_init_range(end_pfn - start_pfn, nid,
>   					zone_id, start_pfn, zone_end_pfn,
>   					MEMINIT_EARLY, NULL, MIGRATE_MOVABLE);
> +
> +		if (hole_pfn < start_pfn)
> +			pgcnt += init_unavailable_range(hole_pfn, start_pfn,
> +							zone_id, nid);
> +		hole_pfn = end_pfn;
>   	}
> +
> +	if (hole_pfn < zone_end_pfn)
> +		pgcnt += init_unavailable_range(hole_pfn, zone_end_pfn,
> +						zone_id, nid);
> +
> +	if (pgcnt)
> +		pr_info("  %s zone: %lld pages in unavailable ranges\n",
> +			zone->name, pgcnt);
>   }
>   
>   static int zone_batchsize(struct zone *zone)
> @@ -6519,8 +6581,19 @@ void __init get_pfn_range_for_nid(unsigned int nid,
>   		*end_pfn = max(*end_pfn, this_end_pfn);
>   	}
>   
> -	if (*start_pfn == -1UL)
> +	if (*start_pfn == -1UL) {
>   		*start_pfn = 0;
> +		return;
> +	}
> +
> +#ifdef CONFIG_SPARSEMEM
> +	/*
> +	 * Sections in the memory map may not match actual populated
> +	 * memory, extend the node span to cover the entire section.
> +	 */
> +	*start_pfn = round_down(*start_pfn, PAGES_PER_SECTION);
> +	*end_pfn = round_up(*end_pfn, PAGES_PER_SECTION);

Does that mean that we might create overlapping zones when one node 
starts in the middle of a section and the other one ends in the middle 
of a section?

Could it be a problem? (e.g., would we have to look at neighboring nodes 
when making the decision to extend, and how far to extend?)

-- 
Thanks,

David / dhildenb