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

[Mike Rapoport]

On Mon, Feb 08, 2021 at 01:11:00PM -0800, Andrew Morton wrote:
> 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?

I don't know how common are such systems, but the bug is exposed only for
builds with DEBUG_VM=y, so after problems with previous versions discovered
by various CI systems I'd say to hold it off till 5.11 is out.

If this time the fix works it'll make it to -stable anyway :)

-- 
Sincerely yours,
Mike.