Re: [PATCH v5 1/1] mm: refactor initialization of struct page for holes in memory layout

[Andrew Morton]

On Mon,  8 Feb 2021 13:08:20 +0200 Mike Rapoport <rppt@xxxxxxxxxx> wrote:

> 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.
> 
> 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.
> 
> ...
>
> 
> Fixes: 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions rather
> that check each PFN")

What are your thoughts on the priority of this (rather large!) fix? 
Are such systems sufficiently common to warrant a 5.11 merge?  -stable?