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:

quoted

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 <redacted>

One nit below, otherwise

Reviewed-by: Mike Rapoport <redacted>

quoted

---
  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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