mm: userfaultfd: remove stale comment about core dump locking

Since commit 7f3bfab ("mm/gup: take mmap_lock in get_dump_page()"),
which landed in v5.10, core dumping doesn't enter fault handling without
holding the mmap_lock anymore.  Remove the stale parts of the comments,
but leave the behavior as-is - letting core dumping block on userfault
handling would be a bad idea and could lead to deadlocks if the dumping
process was handling its own userfaults.

Link: https://lkml.kernel.org/r/20230815212216.264445-1-jannh@google.com
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/userfaultfd.c

@@ -428,15 +428,11 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	 * FOLL_DUMP case, anon memory also checks for FOLL_DUMP with
 	 * the no_page_table() helper in follow_page_mask(), but the
 	 * shmem_vm_ops->fault method is invoked even during
-	 * coredumping without mmap_lock and it ends up here.
+	 * coredumping and it ends up here.
 	 */
 	if (current->flags & (PF_EXITING|PF_DUMPCORE))
 		goto out;
 
-	/*
-	 * Coredumping runs without mmap_lock so we can only check that
-	 * the mmap_lock is held, if PF_DUMPCORE was not set.
-	 */
 	assert_fault_locked(vmf);
 
 	ctx = vma->vm_userfaultfd_ctx.ctx;