Re: [PATCH v6 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout

[David Hildenbrand]

On 22.02.21 11:57, Mike Rapoport 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);
>
> 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.

Does this include pages in the last section has handled by ...
...
> -	/*
> -	 * 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.
> -	 */
> -	pgcnt += init_unavailable_range(PFN_DOWN(next),
> -					round_up(max_pfn, PAGES_PER_SECTION));
> -

^ this code?

Or how is that case handled now?

--
Thanks,

David / dhildenb