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

[David Hildenbrand <david@redhat.com>]

On 12.02.21 10:55, David Hildenbrand wrote:
> 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

s/overlapping zones/overlapping nodes/

> 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