From: Mike Rapoport <rppt@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>,
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>, 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 v1 5/7] mm: introduce page_offline_(begin|end|freeze|unfreeze) to synchronize setting PageOffline()
Date: Wed, 5 May 2021 20:41:12 +0300 [thread overview]
Message-ID: <YJLYuL/EceejLC7L@kernel.org> (raw)
In-Reply-To: <8650f764-8652-a82c-c54f-f67401c800e8@redhat.com>
On Wed, May 05, 2021 at 05:10:33PM +0200, David Hildenbrand wrote:
> On 05.05.21 15:24, Michal Hocko wrote:
> > On Thu 29-04-21 14:25:17, 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|unfreeze) for
> > > synchronizing.
> > >
> > > page_offline_freeze()/page_offline_unfreeze() 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.
> >
> > Please add a note to the PageOffline documentation as well. While are
> > adding the api close enough an explicit note there wouldn't hurt.
>
> Will do.
>
> >
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> >
> > As to the patch itself, I am slightly worried that other pfn walkers
> > might be less tolerant to the locking than the proc ones. On the other
> > hand most users shouldn't really care as they do not tend to touch the
> > memory content and PageOffline check without any synchronization should
> > be sufficient for those. Let's try this out and see where we get...
>
> My thinking. Users that actually read random page content (as discussed in
> the cover letter) are
>
> 1. Hibernation
> 2. Dumping (/proc/kcore, /proc/vmcore)
> 3. Physical memory access bypassing the kernel via /dev/mem
> 4. Live debug tools (kgdb)
I think you can add
5. Very old drivers
> Other PFN walkers really shouldn't (and don't) access random page content.
>
> Thanks!
>
> --
> Thanks,
>
> David / dhildenb
>
>
--
Sincerely yours,
Mike.
next prev parent reply other threads:[~2021-05-05 17:41 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-29 12:25 [PATCH v1 0/7] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages David Hildenbrand
2021-04-29 12:25 ` [PATCH v1 1/7] fs/proc/kcore: drop KCORE_REMAP and KCORE_OTHER David Hildenbrand
2021-05-02 6:31 ` Mike Rapoport
2021-04-29 12:25 ` [PATCH v1 2/7] fs/proc/kcore: pfn_is_ram check only applies to KCORE_RAM David Hildenbrand
2021-05-02 6:31 ` Mike Rapoport
2021-04-29 12:25 ` [PATCH v1 3/7] mm: rename and move page_is_poisoned() David Hildenbrand
2021-05-02 6:32 ` Mike Rapoport
2021-05-05 13:13 ` Michal Hocko
2021-05-05 13:17 ` David Hildenbrand
2021-05-05 13:27 ` Michal Hocko
2021-05-05 13:39 ` David Hildenbrand
2021-05-05 13:45 ` Michal Hocko
2021-05-06 1:08 ` Aili Yao
2021-05-06 0:56 ` Aili Yao
2021-05-06 7:06 ` Michal Hocko
2021-05-06 7:28 ` Aili Yao
2021-05-06 7:55 ` Michal Hocko
2021-05-06 8:52 ` Aili Yao
2021-04-29 12:25 ` [PATCH v1 4/7] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages David Hildenbrand
2021-05-02 6:32 ` Mike Rapoport
2021-04-29 12:25 ` [PATCH v1 5/7] mm: introduce page_offline_(begin|end|freeze|unfreeze) to synchronize setting PageOffline() David Hildenbrand
2021-05-02 6:33 ` Mike Rapoport
2021-05-03 8:11 ` David Hildenbrand
2021-05-05 13:24 ` Michal Hocko
2021-05-05 15:10 ` David Hildenbrand
2021-05-05 17:41 ` Mike Rapoport [this message]
2021-04-29 12:25 ` [PATCH v1 6/7] virtio-mem: use page_offline_(start|end) when " David Hildenbrand
2021-05-02 6:33 ` Mike Rapoport
2021-05-03 8:16 ` David Hildenbrand
2021-05-03 8:23 ` Michael S. Tsirkin
2021-04-29 12:25 ` [PATCH v1 7/7] fs/proc/kcore: use page_offline_(freeze|unfreeze) David Hildenbrand
2021-05-02 6:34 ` Mike Rapoport
2021-05-03 8:28 ` David Hildenbrand
2021-05-03 9:28 ` Mike Rapoport
2021-05-03 10:13 ` David Hildenbrand
2021-05-03 11:33 ` Mike Rapoport
2021-05-03 11:35 ` David Hildenbrand
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