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

From: David Hildenbrand <david@redhat.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Oscar Salvador <osalvador@suse.de>,
	Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>,
	Alex Shi <alex.shi@linux.alibaba.com>,
	Steven Price <steven.price@arm.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Aili Yao <yaoaili@kingsoft.com>, Jiri Bohac <jbohac@suse.cz>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	linux-hyperv@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()
Date: Mon, 17 May 2021 17:18:29 +0200	[thread overview]
Message-ID: <016e96c9-82e6-3259-7a99-8627c3be11c6@redhat.com> (raw)
In-Reply-To: <YKIQfCjq13dSMHOs@kernel.org>

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