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Date:   Wed, 2 Dec 2020 15:47:36 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Mike Rapoport <rppt@kernel.org>
Cc:     linux-mm@kvack.org, Andrea Arcangeli <aarcange@redhat.com>,
        Baoquan He <bhe@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>, Qian Cai <cai@lca.pw>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: refactor initialization of stuct page for holes in
 memory layout
Message-Id: <20201202154736.5799e01b4c27a75b98e863fc@linux-foundation.org>
In-Reply-To: <20201201181502.2340-1-rppt@kernel.org>
References: <20201201181502.2340-1-rppt@kernel.org>
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On Tue,  1 Dec 2020 20:15:02 +0200 Mike Rapoport <rppt@kernel.org> wrote:

> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> 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:?