BACKPORT: userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx

Increments and loads to mmap_changing are always in mmap_lock critical
section.  This ensures that if userspace requests event notification for
non-cooperative operations (e.g.  mremap), userfaultfd operations don't
occur concurrently.

This can be achieved by using a separate read-write semaphore in
userfaultfd_ctx such that increments are done in write-mode and loads in
read-mode, thereby eliminating the dependency on mmap_lock for this
purpose.

This is a preparatory step before we replace mmap_lock usage with per-vma
locks in fill/move ioctls.

Link: https://lkml.kernel.org/r/20240215182756.3448972-3-lokeshgidra@google.com


Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tim Murray <timmurray@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

(cherry picked from commit 5e4c24a5)
Conflicts:
	fs/userfaultfd.c
	include/linux/userfaultfd_k.h
	mm/userfaultfd.c

1. Functions passing control from fs/userfaultfd.c to mm/userfaultfd.c
   were renamed after 6.1.
   a. Replace mfill_atomic_copy() with mcopy_atomic()
   b. Replace mfill_atomic_zeropage() with mfill_zeropage()
   c. Replace mfill_atomic_continue() with mcopy_continue()
   d. Replace mfill_atomic() with __mcopy_atomic()
   e. Replace mfill_atomic_hugetlb() with __mcopy_atomic_hugetlb()
2. uffd flags were unified into a single parameter after 6.1. Replace
   'flags' with 'mcopy_mode' and 'mode'.
3. Fetch dst_mm from dst_vma in __mcopy_atomic_hugetlb().

Bug: 320478828
Change-Id: I77615c36a0c891801c9eb9de3609df4e7f125c39
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>