Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout

[David Hildenbrand <david@redhat.com>]

On 25.02.21 18:06, Mike Rapoport wrote:
> On Thu, Feb 25, 2021 at 04:59:06PM +0100, David Hildenbrand wrote:
>> On 24.02.21 16:39, 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.
>>>
>>> Before commit 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions
>>> rather that check each PFN") the holes inside a zone were re-initialized
>>> during memmap_init() and got their zone/node links right. However, after
>>> that commit nothing updates the struct pages representing such holes.
>>>
>>> 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.
>>>
>>> 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.
>>>
>>> With this change the pages for holes inside a zone will get proper
>>> zone/node links and the pages that are not spanned by any node will get
>>> links to the adjacent zone/node.
>>>
>>> Fixes: 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions rather that check each PFN")
>>> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
>>> Reported-by: Qian Cai <cai@lca.pw>
>>> Reported-by: Andrea Arcangeli <aarcange@redhat.com>
>>> Reviewed-by: Baoquan He <bhe@redhat.com>
>>> ---
>>>    mm/page_alloc.c | 147 +++++++++++++++++++++---------------------------
>>>    1 file changed, 64 insertions(+), 83 deletions(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 3e93f8b29bae..a11a9acde708 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -6280,12 +6280,60 @@ 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);
>>> +	static unsigned long hole_pfn = 0;
>>>    	unsigned long start_pfn, end_pfn;
>>> +	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);
>>> @@ -6295,7 +6343,23 @@ void __meminit __weak memmap_init_zone(struct zone *zone)
>>>    			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;
>>>    	}
>>> +
>>> +#ifdef CONFIG_SPARSEMEM
>>> +	end_pfn = round_up(zone_end_pfn, PAGES_PER_SECTION);
>>> +	if (hole_pfn < end_pfn)
>>> +		pgcnt += init_unavailable_range(hole_pfn, end_pfn,
>>> +						zone_id, nid);
>>
>> We might still double-initialize PFNs when two zones overlap within a
>> section, correct?
> 
> You mean that a section crosses zones boundary?
> I don't think it's that important.
> 
>> This might worth documenting - also, you might want to
>> take some of the original comment the accompanied this code.
> 
> The original comment was not exactly right, I believe the comment above
> init_unavailable_range() better describes what's going on there.

Ah, okay - as long as it's documented I'm happy :)

-- 
Thanks,

David / dhildenb