Re: Arm64 crash while online/offline memory sections

[David Hildenbrand]

On 25.05.21 20:00, Oscar Salvador wrote:
> On Tue, May 25, 2021 at 05:57:34PM +0000, Qian Cai (QUIC) wrote:
> > > Do we know which patch in particular is problematic?
> >
> > Okay, the winner is "mm,memory_hotplug: Allocate memmap from the added memory range".
> >
> > https://lore.kernel.org/linux-mm/20210421102701.25051-5-osalvador@xxxxxxx/
>
> Ok, which means that is irrelevant to having it enabled, as the latter
> patch of that series actualy enables it for arm64.
> Can you work out where exactly the crash happens?
>
> I will have a look into it tomorrow.
>
> Thanks for reporting.

I assume the following will work:

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index b31b3af5c490..6e661d106e96 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -218,14 +218,15 @@ static int memory_block_offline(struct memory_block *mem)
        struct zone *zone;
        int ret;
 
-       zone = page_zone(pfn_to_page(start_pfn));
-
        /*
         * Unaccount before offlining, such that unpopulated zone and kthreads
         * can properly be torn down in offline_pages().
         */
-       if (nr_vmemmap_pages)
+       if (nr_vmemmap_pages) {
+               /* Hotplugged memory has no holes. */
+               zone = page_zone(pfn_to_page(start_pfn));
                adjust_present_page_count(zone, -nr_vmemmap_pages);
+       }
 
        ret = offline_pages(start_pfn + nr_vmemmap_pages,
                            nr_pages - nr_vmemmap_pages);


We must not touch pfn_to_page(start_pfn) if it might be a memory hole.
offline_pages() will make sure there are no holes, but that's too late.

--
Thanks,

David / dhildenb