Re: [PATCH v8 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout

[Andrew Morton]

On Fri, 26 Feb 2021 00:43:51 +0200 Mike Rapoport <rppt@xxxxxxxxxx> wrote:

> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> 
> 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);
> 
> in set_pfnblock_flags_mask() when called with a struct page from a range
> other than E820_TYPE_RAM because there are pages in the range of ZONE_DMA32
> but the unset zone link in struct page makes them appear as a part of
> ZONE_DMA.
> 
> 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. The holes between nodes will be prepended
> to the zone/node above the hole and the trailing pages in the last section
> that will be appended to the zone/node below.
> 
> ...
>
> +#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:
> + * - physical 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 if the
> + *   hole is in the middle of a zone
> + * - zone and node links point to adjacent zone/node if the hole falls on
> + *   the zone boundary; the pages in such holes will be prepended to the
> + *   zone/node above the hole except for the trailing pages in the last
> + *   section that will be appended to the zone/node below.
> + */

The comment helps lot.

>  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;

static implies that pgdat->node_zones[] is alwyas sorted in ascending
pfn order.  Always true?

>  	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 +6348,29 @@ 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
> +	/*
> +	 * Initialize the hole in the range [zone_end_pfn, section_end].
> +	 * If zone boundary falls in the middle of a section, this hole
> +	 * will be re-initialized during the call to this function for the
> +	 * higher zone.
> +	 */
> +	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);
> +#endif
> +
> +	if (pgcnt)
> +		pr_info("  %s zone: %lld pages in unavailable ranges\n",
> +			zone->name, pgcnt);

I'll make that %llu