Re: [PATCH v3 2/2] mm: fix initialization of struct page for holes in memory layout

[David Hildenbrand <david@redhat.com>]

On 11.01.21 20:40, 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 if this page are set to
> default values and the page 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.
> 
> Update init_unavailable_mem() to use zone constraints defined by an
> architecture to properly setup the zone link and use node ID of the
> adjacent range in memblock.memory to set the node link.
> 
> 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>
> ---
>   mm/page_alloc.c | 84 +++++++++++++++++++++++++++++--------------------
>   1 file changed, 50 insertions(+), 34 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index bdbec4c98173..0b56c3ca354e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7077,23 +7077,26 @@ void __init free_area_init_memoryless_node(int nid)
>    * Initialize all valid struct pages in the range [spfn, epfn) and mark them
>    * PageReserved(). Return the number of struct pages that were initialized.
>    */
> -static u64 __init init_unavailable_range(unsigned long spfn, unsigned long epfn)
> +static u64 __init init_unavailable_range(unsigned long spfn, unsigned long epfn,
> +					 int zone, int nid)
>   {
> -	unsigned long pfn;
> +	unsigned long pfn, zone_spfn, zone_epfn;
>   	u64 pgcnt = 0;
>   
> +	zone_spfn = arch_zone_lowest_possible_pfn[zone];
> +	zone_epfn = arch_zone_highest_possible_pfn[zone];
> +
> +	spfn = clamp(spfn, zone_spfn, zone_epfn);
> +	epfn = clamp(epfn, zone_spfn, zone_epfn);
> +
>   	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;
>   		}
> -		/*
> -		 * Use a fake node/zone (0) for now. Some of these pages
> -		 * (in memblock.reserved but not in memblock.memory) will
> -		 * get re-initialized via reserve_bootmem_region() later.
> -		 */
> -		__init_single_page(pfn_to_page(pfn), pfn, 0, 0);
> +
> +		__init_single_page(pfn_to_page(pfn), pfn, zone, nid);
>   		__SetPageReserved(pfn_to_page(pfn));
>   		pgcnt++;
>   	}
> @@ -7102,51 +7105,64 @@ static u64 __init init_unavailable_range(unsigned long spfn, unsigned long epfn)
>   }
>   
>   /*
> - * Only struct pages that are backed by physical memory are zeroed and
> - * initialized by going through __init_single_page(). But, there are some
> - * struct pages which are reserved in memblock allocator and their fields
> - * may be accessed (for example page_to_pfn() on some configuration accesses
> - * flags). We must explicitly initialize those struct pages.
> + * Only struct pages that correspond to ranges defined by memblock.memory
> + * are zeroed and initialized by going through __init_single_page() during
> + * memmap_init().
> + *
> + * 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
>    *
> - * This function also addresses a similar issue where struct pages are left
> - * uninitialized because the physical address range is not covered by
> - * memblock.memory or memblock.reserved. That could happen when memblock
> - * layout is manually configured via memmap=, or when the highest physical
> - * address (max_pfn) does not end on a section boundary.
> + * Explicitly initialize those struct pages so that:
> + * - PG_Reserved is set
> + * - zone link is set accorging to the architecture constrains
> + * - node is set to node id of the next populated region except for the
> + *   trailing hole where last node id is used
>    */
> -static void __init init_unavailable_mem(void)
> +static void __init init_zone_unavailable_mem(int zone)
>   {
> -	phys_addr_t start, end;
> -	u64 i, pgcnt;
> -	phys_addr_t next = 0;
> +	unsigned long start, end;
> +	int i, nid;
> +	u64 pgcnt;
> +	unsigned long next = 0;
>   
>   	/*
> -	 * Loop through unavailable ranges not covered by memblock.memory.
> +	 * Loop through holes in memblock.memory and initialize struct
> +	 * pages corresponding to these holes
>   	 */
>   	pgcnt = 0;
> -	for_each_mem_range(i, &start, &end) {
> +	for_each_mem_pfn_range(i, MAX_NUMNODES, &start, &end, &nid) {
>   		if (next < start)
> -			pgcnt += init_unavailable_range(PFN_DOWN(next),
> -							PFN_UP(start));
> +			pgcnt += init_unavailable_range(next, start, zone, nid);
>   		next = end;
>   	}
>   
>   	/*
> -	 * Early sections always have a fully populated memmap for the whole
> -	 * section - see pfn_valid(). If the last section has holes at the
> -	 * end and that section is marked "online", the memmap will be
> -	 * considered initialized. Make sure that memmap has a well defined
> -	 * state.
> +	 * Last section may surpass the actual end of memory (e.g. we can
> +	 * have 1Gb section and 512Mb of RAM pouplated).
> +	 * Make sure that memmap has a well defined state in this case.
>   	 */
> -	pgcnt += init_unavailable_range(PFN_DOWN(next),
> -					round_up(max_pfn, PAGES_PER_SECTION));
> +	end = round_up(max_pfn, PAGES_PER_SECTION);
> +	pgcnt += init_unavailable_range(next, end, zone, nid);
>   
>   	/*
>   	 * Struct pages that do not have backing memory. This could be because
>   	 * firmware is using some of this memory, or for some other reasons.
>   	 */
>   	if (pgcnt)
> -		pr_info("Zeroed struct page in unavailable ranges: %lld pages", pgcnt);
> +		pr_info("Zone %s: zeroed struct page in unavailable ranges: %lld pages", zone_names[zone], pgcnt);
> +}
> +
> +static void __init init_unavailable_mem(void)
> +{
> +	int zone;
> +
> +	for (zone = 0; zone < ZONE_MOVABLE; zone++)
> +		init_zone_unavailable_mem(zone);

Why < ZONE_MOVABLE?

I remember we can have memory holes inside the movable zone when messing 
with "movablecore" cmdline parameter.


-- 
Thanks,

David / dhildenb