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

[David Hildenbrand]

On 25.02.21 23:43, 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);
>
> 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.
>
> Fixes: 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions rather that check each PFN")
> Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Reported-by: Qian Cai <cai@xxxxxx>
> Reported-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Reviewed-by: Baoquan He <bhe@xxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks Mike!


--
Thanks,

David / dhildenb