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

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Date:   Fri, 18 Oct 2019 13:34:37 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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 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.
-- 
Michal Hocko
SUSE Labs