Re: [Bug 206401] kernel panic on Hyper-V after 5 minutes due to memory hot-add

[Baoquan He <bhe@redhat.com>]

On 02/17/20 at 11:24am, David Hildenbrand wrote:
> On 17.02.20 11:13, Baoquan He wrote:
> > On 02/17/20 at 10:34am, Oscar Salvador wrote:
> >> On Mon, Feb 17, 2020 at 02:46:27PM +0900, kkabe@vega.pgw.jp wrote:
> >>> ===========================================
> >>> struct page * __meminit populate_section_memmap(unsigned long pfn,
> >>>                 unsigned long nr_pages, int nid, struct vmem_altmap *altmap)
> >>> {
> >>>         struct page *page, *ret;
> >>>         unsigned long memmap_size = sizeof(struct page) * PAGES_PER_SECTION;
> >>>
> >>>         page = alloc_pages(GFP_KERNEL|__GFP_NOWARN, get_order(memmap_size));
> >>>         if (page) {
> >>>                 goto got_map_page;
> >>>         }
> >>> pr_info("%s: alloc_pages() returned 0x%p (should be 0), reverting to vmalloc(memmap_size=%lu)\n", __func__, page, memmap_size);
> >>> BUG_ON(page != 0);
> >>>
> >>>         ret = vmalloc(memmap_size);
> >>> pr_info("%s: vmalloc(%lu) returned 0x%p\n", __func__, memmap_size, ret);
> >>>         if (ret) {
> >>>                 goto got_map_ptr;
> >>>         }
> >>>
> >>>         return NULL;
> >>> got_map_page:
> >>>         ret = (struct page *)pfn_to_kaddr(page_to_pfn(page));
> >>> pr_info("%s: allocated struct page *page=0x%p\n", __func__, page);
> >>> got_map_ptr:
> >>>
> >>> pr_info("%s: returning struct page * =0x%p\n", __func__, ret);
> >>>         return ret;
> >>> }
> >>
> >> Could you please replace %p with %px. Wih the first, pointers are hashed so it is trickier
> >> to get an overview of the meaning.
> >>
> >> David could be right about ZONE_NORMAL vs ZONE_HIGHMEM.
> >> IIUC, default_kernel_zone_for_pfn and default_zone_for_pfn seem to only deal with
> >> (ZONE_DMA,ZONE_NORMAL] or ZONE_MOVABLE.
> > 
> > Ah, I think you both have spotted the problem.
> >  
> > In i386, if w/o momory hot add, normal memory will only include those
> > below 896M and they are added into normal zone. The left are added into
> > highmem zone.
> >  
> > How this influence the page allocation?
> >  
> > Very huge. As we know, in i386, normal memory can be accessed with
> > virt_to_phys, namely PAGE_OFFSET + phys. But highmem has to be accessed
> > with kmap. However, the later hot added memory are all put into normal
> > memmory, accessing into them will stump into vmalloc area, I would say.
> >  
> > So, i386 doesn't support memory hot add well.  Not sure if below change
> > can make it work normally.
> >  
> > We can just adjus the hot adding code as we have done for boot memmory.
> > Iterate zone from highmem if allowed when hot add memory.
> >  
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 475d0d68a32c..1380392d9ef5 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -716,7 +716,10 @@ static struct zone *default_kernel_zone_for_pfn(int nid, unsigned long start_pfn
> >  	struct pglist_data *pgdat = NODE_DATA(nid);
> >  	int zid;
> >  
> > -	for (zid = 0; zid <= ZONE_NORMAL; zid++) {
> > +	for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> 
> ZONE_DEVICE? :/

Not sure if ZONE_DEVICE will be supported on 32 bit system.


> 
> > +		if (zid == ZONE_MOVABLE)
> > +			continue;
> > +
> >  		struct zone *zone = &pgdat->node_zones[zid];
> >  
> >  		if (zone_intersects(zone, start_pfn, nr_pages))
> > 
> > 
> 
> What if somebody onlines memory from user space explicitly to the normal
> zone? We can trigger crashes?

Seems the current i386 code doesn't support it. Unless we change that
too. If not reserving virtual address space, later added any memory has
to be highmem.

> 
> This doesn't look like it ever worked reliably, can we just disable
> memory hotplug in case we have PAE? (especially, as continued i386
> support is questionable)

This is not PAE, this is only HIGHMEM4G.