mm/userfaultfd: fix release hang over concurrent GUP

This patch should fix a possible userfaultfd release() hang during
concurrent GUP.

This problem was initially reported by Dimitris Siakavaras in July 2023
[1] in a firecracker use case.  Firecracker has a separate process
handling page faults remotely, and when the process releases the
userfaultfd it can race with a concurrent GUP from KVM trying to fault in
a guest page during the secondary MMU page fault process.

A similar problem was reported recently again by Jinjiang Tu in March 2025
[2], even though the race happened this time with a mlockall() operation,
which does GUP in a similar fashion.

In 2017, commit 656710a6 ("userfaultfd: non-cooperative: closing the
uffd without triggering SIGBUS") was trying to fix this issue.  AFAIU,
that fixes well the fault paths but may not work yet for GUP.  In GUP, the
issue is NOPAGE will be almost treated the same as "page fault resolved"
in faultin_page(), then the GUP will follow page again, seeing page
missing, and it'll keep going into a live lock situation as reported.

This change makes core mm return RETRY instead of NOPAGE for both the GUP
and fault paths, proactively releasing the mmap read lock.  This should
guarantee the other release thread make progress on taking the write lock
and avoid the live lock even for GUP.

When at it, rearrange the comments to make sure it's uptodate.

[1] https://lore.kernel.org/r/79375b71-db2e-3e66-346b-254c90d915e2@cslab.ece.ntua.gr
[2] https://lore.kernel.org/r/20250307072133.3522652-1-tujinjiang@huawei.com

Link: https://lkml.kernel.org/r/20250312145131.1143062-1-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Jinjiang Tu <tujinjiang@huawei.com>
Cc: Dimitris Siakavaras <jimsiak@cslab.ece.ntua.gr>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>