Re: [PATCH] mm/hotplug: invalid PFNs from pfn_to_online_page()

[Qian Cai <cai@lca.pw>]

On 1/21/19 11:38 AM, Qian Cai wrote:
> 
> 
> On 1/21/19 4:53 AM, Michal Hocko wrote:
>> On Thu 17-01-19 21:16:50, Qian Cai wrote:
>>> On an arm64 ThunderX2 server, the first kmemleak scan would crash [1]
>>> with CONFIG_DEBUG_VM_PGFLAGS=y due to page_to_nid() found a pfn that is
>>> not directly mapped (MEMBLOCK_NOMAP). Hence, the page->flags is
>>> uninitialized.
>>>
>>> This is due to the commit 9f1eb38e0e11 ("mm, kmemleak: little
>>> optimization while scanning") starts to use pfn_to_online_page() instead
>>> of pfn_valid(). However, in the CONFIG_MEMORY_HOTPLUG=y case,
>>> pfn_to_online_page() does not call memblock_is_map_memory() while
>>> pfn_valid() does.
>>
>> How come there is an online section which has an pfn_valid==F? We do
>> allocate the full section worth of struct pages so there is a valid
>> struct page. Is there any hole inside this section?
> 
> It has CONFIG_HOLES_IN_ZONE=y.

Actually, this does not seem have anything to do with holes.

68709f45385a arm64: only consider memblocks with NOMAP cleared for linear mapping

This causes pages marked as nomap being no long reassigned to the new zone in
memmap_init_zone() by calling __init_single_page().

There is an old discussion for this topic.
https://lkml.org/lkml/2016/11/30/566