mm, mremap: fix mremap() expanding for vma's with vm_ops->close()

Fabian has reported another regression in 6.1 due to ca3d76b ("mm:
add merging after mremap resize").  The problem is that vma_merge() can
fail when vma has a vm_ops->close() method, causing is_mergeable_vma()
test to be negative.  This was happening for vma mapping a file from
fuse-overlayfs, which does have the method.  But when we are simply
expanding the vma, we never remove it due to the "merge" with the added
area, so the test should not prevent the expansion.

As a quick fix, check for such vmas and expand them using vma_adjust()
directly as was done before commit ca3d76b.  For a more robust long
term solution we should try to limit the check for vma_ops->close only to
cases that actually result in vma removal, so that no merge would be
prevented unnecessarily.

[[email protected]: fix indenting whitespace, reflow comment]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: ca3d76b ("mm: add merging after mremap resize")
Signed-off-by: Vlastimil Babka <[email protected]>
Reported-by: Fabian Vogt <[email protected]>
  Link: https://bugzilla.suse.com/show_bug.cgi?id=1206359#c35
Tested-by: Fabian Vogt <[email protected]>
Cc: Jakub Matěna <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/mremap.c

@@ -1027,16 +1027,29 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 			}
 
 			/*
-			 * Function vma_merge() is called on the extension we are adding to
-			 * the already existing vma, vma_merge() will merge this extension with
-			 * the already existing vma (expand operation itself) and possibly also
-			 * with the next vma if it becomes adjacent to the expanded vma and
-			 * otherwise compatible.
+			 * Function vma_merge() is called on the extension we
+			 * are adding to the already existing vma, vma_merge()
+			 * will merge this extension with the already existing
+			 * vma (expand operation itself) and possibly also with
+			 * the next vma if it becomes adjacent to the expanded
+			 * vma and  otherwise compatible.
+			 *
+			 * However, vma_merge() can currently fail due to
+			 * is_mergeable_vma() check for vm_ops->close (see the
+			 * comment there). Yet this should not prevent vma
+			 * expanding, so perform a simple expand for such vma.
+			 * Ideally the check for close op should be only done
+			 * when a vma would be actually removed due to a merge.
 			 */
-			vma = vma_merge(mm, vma, extension_start, extension_end,
+			if (!vma->vm_ops || !vma->vm_ops->close) {
+				vma = vma_merge(mm, vma, extension_start, extension_end,
 					vma->vm_flags, vma->anon_vma, vma->vm_file,
 					extension_pgoff, vma_policy(vma),
 					vma->vm_userfaultfd_ctx, anon_vma_name(vma));
+			} else if (vma_adjust(vma, vma->vm_start, addr + new_len,
+				   vma->vm_pgoff, NULL)) {
+				vma = NULL;
+			}
 			if (!vma) {
 				vm_unacct_memory(pages);
 				ret = -ENOMEM;