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 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? This might worth documenting - also, you might want to 
take some of the original comment the accompanied this code.

You should also document (in the patch description?) that node/zone 
spans are not properly handled yet for such hole pfns and that this 
might require care in the future.


I played a little with weird setups and expected the memap state using 
page-types (well, I can't inspect the node/zone that way but at least 
have a look if the memmap was initialized). No surprises.

-- 
Thanks,

David / dhildenb