Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout

[Mike Rapoport]

On Thu, Feb 25, 2021 at 04:59:06PM +0100, David Hildenbrand wrote:
> On 24.02.21 16:39, Mike Rapoport 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 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.
> > 
> > Fixes: 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions rather that check each PFN")
> > Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > Reported-by: Qian Cai <cai@xxxxxx>
> > Reported-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Reviewed-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> >   mm/page_alloc.c | 147 +++++++++++++++++++++---------------------------
> >   1 file changed, 64 insertions(+), 83 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3e93f8b29bae..a11a9acde708 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -6280,12 +6280,60 @@ static void __meminit zone_init_free_lists(struct zone *zone)
> >   	}
> >   }
> > +#if !defined(CONFIG_FLAT_NODE_MEM_MAP)
> > +/*
> > + * Only struct pages that correspond to ranges defined by memblock.memory
> > + * are zeroed and initialized by going through __init_single_page() during
> > + * memmap_init_zone().
> > + *
> > + * But, there could be struct pages that correspond to holes in
> > + * memblock.memory. This can happen because of the following reasons:
> > + * - phyiscal memory bank size is not necessarily the exact multiple of the
> > + *   arbitrary section size
> > + * - early reserved memory may not be listed in memblock.memory
> > + * - memory layouts defined with memmap= kernel parameter may not align
> > + *   nicely with memmap sections
> > + *
> > + * Explicitly initialize those struct pages so that:
> > + * - PG_Reserved is set
> > + * - zone and node links point to zone and node that span the page
> > + */
> > +static u64 __meminit init_unavailable_range(unsigned long spfn,
> > +					    unsigned long epfn,
> > +					    int zone, int node)
> > +{
> > +	unsigned long pfn;
> > +	u64 pgcnt = 0;
> > +
> > +	for (pfn = spfn; pfn < epfn; pfn++) {
> > +		if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages))) {
> > +			pfn = ALIGN_DOWN(pfn, pageblock_nr_pages)
> > +				+ pageblock_nr_pages - 1;
> > +			continue;
> > +		}
> > +		__init_single_page(pfn_to_page(pfn), pfn, zone, node);
> > +		__SetPageReserved(pfn_to_page(pfn));
> > +		pgcnt++;
> > +	}
> > +
> > +	return pgcnt;
> > +}
> > +#else
> > +static inline u64 init_unavailable_range(unsigned long spfn, unsigned long epfn,
> > +					 int zone, int node)
> > +{
> > +	return 0;
> > +}
> > +#endif
> > +
> >   void __meminit __weak memmap_init_zone(struct zone *zone)
> >   {
> >   	unsigned long zone_start_pfn = zone->zone_start_pfn;
> >   	unsigned long zone_end_pfn = zone_start_pfn + zone->spanned_pages;
> >   	int i, nid = zone_to_nid(zone), zone_id = zone_idx(zone);
> > +	static unsigned long hole_pfn = 0;
> >   	unsigned long start_pfn, end_pfn;
> > +	u64 pgcnt = 0;
> >   	for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, NULL) {
> >   		start_pfn = clamp(start_pfn, zone_start_pfn, zone_end_pfn);
> > @@ -6295,7 +6343,23 @@ void __meminit __weak memmap_init_zone(struct zone *zone)
> >   			memmap_init_range(end_pfn - start_pfn, nid,
> >   					zone_id, start_pfn, zone_end_pfn,
> >   					MEMINIT_EARLY, NULL, MIGRATE_MOVABLE);
> > +
> > +		if (hole_pfn < start_pfn)
> > +			pgcnt += init_unavailable_range(hole_pfn, start_pfn,
> > +							zone_id, nid);
> > +		hole_pfn = 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);
> 
> We might still double-initialize PFNs when two zones overlap within a
> section, correct?

You mean that a section crosses zones boundary?
I don't think it's that important.

> This might worth documenting - also, you might want to
> take some of the original comment the accompanied this code.

The original comment was not exactly right, I believe the comment above
init_unavailable_range() better describes what's going on there.
 
> You should also document (in the patch description?) that node/zone spans
> are not properly handled yet for such hole pfns and that this might require
> care in the future.

I think Link: will suffice for this.
 
> I played a little with weird setups and expected the memap state using
> page-types (well, I can't inspect the node/zone that way but at least have a
> look if the memmap was initialized). No surprises.

Great, thanks!

-- 
Sincerely yours,
Mike.