linux-kernel - Re: [PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()

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Date:   Mon, 17 May 2021 17:18:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Steven Price <steven.price@....com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Aili Yao <yaoaili@...gsoft.com>, Jiri Bohac <jbohac@...e.cz>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-hyperv@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw)
 to synchronize setting PageOffline()

On 17.05.21 08:43, Mike Rapoport wrote:
> On Fri, May 14, 2021 at 07:22:45PM +0200, David Hildenbrand wrote:
>> A driver might set a page logically offline -- PageOffline() -- and
>> turn the page inaccessible in the hypervisor; after that, access to page
>> content can be fatal. One example is virtio-mem; while unplugged memory
>> -- marked as PageOffline() can currently be read in the hypervisor, this
>> will no longer be the case in the future; for example, when having
>> a virtio-mem device backed by huge pages in the hypervisor.
>>
>> Some special PFN walkers -- i.e., /proc/kcore -- read content of random
>> pages after checking PageOffline(); however, these PFN walkers can race
>> with drivers that set PageOffline().
>>
>> Let's introduce page_offline_(begin|end|freeze|thaw) for
>> synchronizing.
>>
>> page_offline_freeze()/page_offline_thaw() allows for a subsystem to
>> synchronize with such drivers, achieving that a page cannot be set
>> PageOffline() while frozen.
>>
>> page_offline_begin()/page_offline_end() is used by drivers that care about
>> such races when setting a page PageOffline().
>>
>> For simplicity, use a rwsem for now; neither drivers nor users are
>> performance sensitive.
>>
>> Acked-by: Michal Hocko <mhocko@...e.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> One nit below, otherwise
> 
> Reviewed-by: Mike Rapoport <rppt@...ux.ibm.com>
> 
>> ---
>>   include/linux/page-flags.h | 10 ++++++++++
>>   mm/util.c                  | 40 ++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 50 insertions(+)
>>
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index daed82744f4b..ea2df9a247b3 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -769,9 +769,19 @@ PAGE_TYPE_OPS(Buddy, buddy)
>>    * relies on this feature is aware that re-onlining the memory block will
>>    * require to re-set the pages PageOffline() and not giving them to the
>>    * buddy via online_page_callback_t.
>> + *
>> + * There are drivers that mark a page PageOffline() and do not expect any
> 
> Maybe "and expect there won't be any further access"...
> 

Thanks, makes sense.

I'll wait a bit before I resend.

-- 
Thanks,

David / dhildenb