The basic idea here is to "simulate" memory poisoning for VMs.  A VM
running on some host might encounter a memory error, after which some
page(s) are poisoned (i.e., future accesses SIGBUS).  They expect that
once poisoned, pages can never become "un-poisoned".  So, when we live
migrate the VM, we need to preserve the poisoned status of these pages.

When live migrating, we try to get the guest running on its new host as
quickly as possible.  So, we start it running before all memory has been
copied, and before we're certain which pages should be poisoned or not.

So the basic way to use this new feature is:

- On the new host, the guest's memory is registered with userfaultfd, in
  either MISSING or MINOR mode (doesn't really matter for this purpose).
- On any first access, we get a userfaultfd event. At this point we can
  communicate with the old host to find out if the page was poisoned.
- If so, we can respond with a UFFDIO_POISON - this places a swap marker
  so any future accesses will SIGBUS. Because the pte is now "present",
  future accesses won't generate more userfaultfd events, they'll just
  SIGBUS directly.

UFFDIO_POISON does not handle unmapping previously-present PTEs.  This
isn't needed, because during live migration we want to intercept all
accesses with userfaultfd (not just writes, so WP mode isn't useful for
this).  So whether minor or missing mode is being used (or both), the PTE
won't be present in any case, so handling that case isn't needed.

Similarly, UFFDIO_POISON won't replace existing PTE markers.  This might
be okay to do, but it seems to be safer to just refuse to overwrite any
existing entry (like a UFFD_WP PTE marker).

Link: https://lkml.kernel.org/r/20230707215540.2324998-5-axelrasmussen@google.comSigned-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gaosheng Cui <cuigaosheng1@huawei.com>
Cc: Huang, Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nadav Amit <namit@vmware.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>