linux-kernel - Re: memory offline infinite loop after soft offline

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Date:   Fri, 18 Oct 2019 13:51:12 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>, Qian Cai <cai@....pw>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: memory offline infinite loop after soft offline

On 18.10.19 13:34, Michal Hocko wrote:
> On Fri 18-10-19 13:00:45, David Hildenbrand wrote:
>> 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)));
> 
> OK, I see what you mean now. In other words similar to eeb0efd071d82.
> 

Exactly.

-- 

Thanks,

David / dhildenb