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

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Date:   Fri, 18 Oct 2019 13:00:45 +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 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