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 06:13pm, 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

Hmm, here I mean __pa and __va.

> 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++) {
> +		if (zid == ZONE_MOVABLE)
> +			continue;
> +
>  		struct zone *zone = &pgdat->node_zones[zid];
>  
>  		if (zone_intersects(zone, start_pfn, nr_pages))
> 
>