linux-kernel - Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with a reference count of 0

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Date:   Wed, 16 Oct 2019 15:45:06 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Juergen Gross <jgross@...e.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pingfan Liu <kernelfans@...il.com>, Qian Cai <cai@....pw>,
        Dan Williams <dan.j.williams@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Wei Yang <richardw.yang@...ux.intel.com>,
        Alexander Potapenko <glider@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yu Zhao <yuzhao@...gle.com>, Minchan Kim <minchan@...nel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with
 a reference count of 0

On 16.10.19 13:43, Michal Hocko wrote:
> On Thu 19-09-19 16:22:25, David Hildenbrand wrote:
>> virtio-mem wants to allow to offline memory blocks of which some parts
>> were unplugged, especially, to later offline and remove completely
>> unplugged memory blocks. The important part is that PageOffline() has
>> to remain set until the section is offline, so these pages will never
>> get accessed (e.g., when dumping). The pages should not be handed
>> back to the buddy (which would require clearing PageOffline() and
>> result in issues if offlining fails and the pages are suddenly in the
>> buddy).
>>
>> Let's use "PageOffline() + reference count = 0" as a sign to
>> memory offlining code that these pages can simply be skipped when
>> offlining, similar to free or HWPoison pages.
>>
>> Pass flags to test_pages_isolated(), similar as already done for
>> has_unmovable_pages(). Use a new flag to indicate the
>> requirement of memory offlining to skip over these special pages.
>>
>> In has_unmovable_pages(), make sure the pages won't be detected as
>> movable. This is not strictly necessary, however makes e.g.,
>> alloc_contig_range() stop early, trying to isolate such page blocks -
>> compared to failing later when testing if all pages were isolated.
>>
>> Also, make sure that when a reference to a PageOffline() page is
>> dropped, that the page will not be returned to the buddy.
>>
>> memory devices (like virtio-mem) that want to make use of this
>> functionality have to make sure to synchronize against memory offlining,
>> using the memory hotplug notifier.
>>
>> Alternative: Allow to offline with a reference count of 1
>> and use some other sign in the struct page that offlining is permitted.
> 
> Few questions. I do not see onlining code to take care of this special
> case. What should happen when offline && online?
> Should we allow to try_remove_memory to succeed with these pages?
> Do we really have hook into __put_page? Why do we even care about the
> reference count of those pages?

Oh, I forgot to answer this questions. The __put_page() change is 
necessary for the following race I identified:

Page has a refcount of 1 (e.g., allocated by virtio-mem using 
alloc_contig_range()).

a) kernel: get_page_unless_zero(page): refcount = 2
b) virtio-mem: set page PG_offline, reduce refcount): refocunt = 1
c) kernel: put_page(page): refcount = 0

The page would suddenly be given to the buddy. which is bad.

-- 

Thanks,

David / dhildenb