Re: [PATCH] mm: refactor initialization of stuct page for holes in memory layout

[Andrew Morton]

On Tue,  1 Dec 2020 20:15:02 +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 iterated 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 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);

That sounds pretty serious.

> because there are pages in both ZONE_DMA32 and ZONE_DMA (unset zone link in
> struct page) in the same pageblock.
> 
> Interleave initialization of pages that correspond to holes with the
> initialization of memory map, so that zone and node information will be
> properly set on such pages.
> 

Should this be backported to -stable?  If so, do we have a suitable Fixes:?