userfaultfd: fix move_pages_pte() splitting folio under RCU read lock

While testing the split PMD path with lockdep enabled I've got an "Invalid
wait context" error caused by split_huge_page_to_list() trying to lock
anon_vma->rwsem while inside RCU read section.  The issues is due to
move_pages_pte() calling split_folio() under RCU read lock.  Fix this by
unmapping the PTEs and exiting RCU read section before splitting the folio
and then retrying.  The same retry pattern is used when locking the folio
or anon_vma in this function.  After splitting the large folio we unlock
and release it because after the split the old folio might not be the one
that contains the src_addr.

Link: https://lkml.kernel.org/r/20240102233256.1077959-1-surenb@google.com
Fixes: adef4406 ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/userfaultfd.c

@@ -1078,9 +1078,18 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 
 		/* at this point we have src_folio locked */
 		if (folio_test_large(src_folio)) {
+			/* split_folio() can block */
+			pte_unmap(&orig_src_pte);
+			pte_unmap(&orig_dst_pte);
+			src_pte = dst_pte = NULL;
 			err = split_folio(src_folio);
 			if (err)
 				goto out;
+			/* have to reacquire the folio after it got split */
+			folio_unlock(src_folio);
+			folio_put(src_folio);
+			src_folio = NULL;
+			goto retry;
 		}
 
 		if (!src_anon_vma) {