Re: memory offline infinite loop after soft offline

David Hildenbrand
2019-10-18T11:00:49Z

On 18.10.19 10:55, Michal Hocko wrote:
> On Fri 18-10-19 10:38:21, David Hildenbrand wrote:
>> On 18.10.19 10:24, Michal Hocko wrote:
>>> On Fri 18-10-19 10:13:36, David Hildenbrand wrote:
>>> [...]
>>>> However, if the compound page spans multiple pageblocks
>>>
>>> Although hugetlb pages spanning pageblocks are possible this shouldn't
>>> matter in__test_page_isolated_in_pageblock because this function doesn't
>>> really operate on pageblocks as the name suggests. It is simply
>>> traversing all valid RAM ranges (see walk_system_ram_range).
>>
>> As long as the hugepages don't span memory blocks/sections, you are right. I
>> have no experience with gigantic pages in this regard.
>
> They can clearly span sections (1GB is larger than 128MB). Why do you
> think it matters actually? walk_system_ram_range walks RAM ranges and no
> allocation should span holes in RAM right?
>

Let's explore what I was thinking. If we can agree that any compound
page is always aligned to its size , then what I tell here is not
applicable. I know it is true for gigantic pages.

Some extreme example to clarify

[ memory block 0 (128MB) ][ memory block 1 (128MB) ]
[ compound page (128MB) ]

If you would offline memory block 1, and you detect PG_offline on the
first page of that memory block (PageHWPoison(compound_head(page))), you
would jump over the whole memory block (pfn += 1 <<
compound_order(page)), leaving 64MB of the memory block unchecked.

Again, if any compound page has the alignment restrictions (PFN of head
aligned to 1 << compound_order(page)), this is not possible.

If it is, however, possible, the "clean" thing would be to only jump
over the remaining part of the compound page, e.g., something like

pfn += (1 << compound_order(page)) - (page - compound_head(page)));

--

Thanks,

David / dhildenb