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

From: Andrew Morton <akpm@linux-foundation.org>
To: Mike Rapoport <rppt@kernel.org>
Cc: "Andrea Arcangeli" <aarcange@redhat.com>,
	"Baoquan He" <bhe@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
	"Chris Wilson" <chris@chris-wilson.co.uk>,
	"David Hildenbrand" <david@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Łukasz Majczak" <lma@semihalf.com>,
	"Mel Gorman" <mgorman@suse.de>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Mike Rapoport" <rppt@linux.ibm.com>, "Qian Cai" <cai@lca.pw>,
	"Sarvela, Tomi P" <tomi.p.sarvela@intel.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	stable@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH v5 1/1] mm: refactor initialization of struct page for holes in memory layout
Date: Mon, 8 Feb 2021 13:11:00 -0800	[thread overview]
Message-ID: <20210208131100.7273d249a5d00cac0d247fcf@linux-foundation.org> (raw)
In-Reply-To: <20210208110820.6269-1-rppt@kernel.org>

On Mon,  8 Feb 2021 13:08:20 +0200 Mike Rapoport <rppt@kernel.org> 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?