[PATCH stable 5.13.y/5.12.y 0/2] mm/thp: Fix uffd-wp with fork(); crash on pmd migration entry on fork

[Peter Xu <peterx@redhat.com>]

In summary: this series should be needed for 5.10/5.12/5.13. This is the
5.13.y/5.12.y backport of the series, and it should be able to be applied on
both of the branches.  Patch 1 is a dependency of patch 2, while patch 2 should
be the real fix.

This series should be able to fix a rare race that mentioned in thread:

https://lore.kernel.org/linux-mm/796cbb7-5a1c-1ba0-dde5-479aba8224f2@google.com/

This fact wasn't discovered when the fix got proposed and merged, because the
fix was originally about uffd-wp and its fork event.  However it turns out that
the problematic commit b569a1760782f3d is also causing crashing on fork() of
pmd migration entries which is even more severe than the original uffd-wp
problem.

Stable kernels at least on 5.12.y has the crash reproduced, and it's possible
5.13.y and 5.10.y could hit it due to having the problematic commit
b569a1760782f3d but lacking of the uffd-wp fix patch (8f34f1eac382, which is
also patch 2 of this series).

The pmd entry crash problem was reported by Igor Raits <igor@gooddata.com> and
debugged by Hugh Dickins <hughd@google.com>.

Please review, thanks.

Peter Xu (2):
  mm/thp: simplify copying of huge zero page pmd when fork
  mm/userfaultfd: fix uffd-wp special cases for fork()

 include/linux/huge_mm.h |  2 +-
 include/linux/swapops.h |  2 ++
 mm/huge_memory.c        | 36 +++++++++++++++++-------------------
 mm/memory.c             | 25 +++++++++++++------------
 4 files changed, 33 insertions(+), 32 deletions(-)

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2.31.1