Re: [PATCH v6 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout

[Mike Rapoport <rppt@kernel.org>]

On Tue, Feb 23, 2021 at 10:49:44AM +0100, David Hildenbrand wrote:
> On 23.02.21 10:48, Mike Rapoport wrote:
> > On Tue, Feb 23, 2021 at 09:04:19AM +0100, David Hildenbrand wrote:
> > > On 22.02.21 11:57, Mike Rapoport 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 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.
> > > > 
> > > > Before commit 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions
> > > > rather that check each PFN") the holes inside a zone were re-initialized
> > > > during memmap_init() and got their zone/node links right. However, after
> > > > that commit nothing updates the struct pages representing such holes.
> > > > 
> > > > 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.
> > > > 
> > > > Interleave initialization of the unavailable pages with the normal
> > > > initialization of memory map, so that zone and node information will be
> > > > properly set on struct pages that are not backed by the actual memory.
> > > > 
> > > > With this change the pages for holes inside a zone will get proper
> > > > zone/node links and the pages that are not spanned by any node will get
> > > > links to the adjacent zone/node.
> > > 
> > > Does this include pages in the last section has handled by ...
> > > ...
> > > > -	/*
> > > > -	 * Early sections always have a fully populated memmap for the whole
> > > > -	 * section - see pfn_valid(). If the last section has holes at the
> > > > -	 * end and that section is marked "online", the memmap will be
> > > > -	 * considered initialized. Make sure that memmap has a well defined
> > > > -	 * state.
> > > > -	 */
> > > > -	pgcnt += init_unavailable_range(PFN_DOWN(next),
> > > > -					round_up(max_pfn, PAGES_PER_SECTION));
> > > > -
> > > 
> > > ^ this code?
> > > 
> > > Or how is that case handled now?
> > 
> > Hmm, now it's clamped to node_end_pfn/zone_end_pfn, so in your funny example with
> > 
> >      -object memory-backend-ram,id=bmem0,size=4160M \
> >      -object memory-backend-ram,id=bmem1,size=4032M \
> > 
> > this is not handled :(
> > 
> > But it will be handled with this on top:
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 29bbd08b8e63..6c9b490f5a8b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -6350,9 +6350,12 @@ void __meminit __weak memmap_init_zone(struct zone *zone)
> >   		hole_pfn = end_pfn;
> >   	}
> > -	if (hole_pfn < zone_end_pfn)
> > -		pgcnt += init_unavailable_range(hole_pfn, zone_end_pfn,
> > +#ifdef CONFIG_SPARSEMEM
> > +	end_pfn = round_up(zone_end_pfn, PAGES_PER_SECTION);
> > +	if (hole_pfn < end_pfn)
> > +		pgcnt += init_unavailable_range(hole_pfn, end_pfn,
> >   						zone_id, nid);
> > +#endif
> >   	if (pgcnt)
> >   		pr_info("  %s zone: %lld pages in unavailable ranges\n",
> > 
> 
> 
> Also, just wondering, will PFN 0 still get initialized?

Yes, it gets 0,0 links, but it is still outside node/zone span.

-- 
Sincerely yours,
Mike.