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SubjectRe: [PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()
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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@suse.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>
> One nit below, otherwise
>
> Reviewed-by: Mike Rapoport <rppt@linux.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

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