[PATCH v2 2/2] mm: mremap: unlink anon_vmas when mremap with MREMAP_DONTUNMAP success

[Li Xinhai <lixinhai.lxh@gmail.com>]

mremap with MREMAP_DONTUNMAP can move all page table entries to new vma,
which means all pages allocated for the old vma are not relevant to it
anymore, and the relevant anon_vma links needs to be unlinked, in nature
the old vma is much like been freshly created and have no pages been
fault in.

But we should not do unlink, if the new vma has effectively merged with
the old one.

Cc: Brian Geffon <bgeffon@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
---
v1->v2:
Need to consider the old vma just been partially moved, if so we must
keep the anon_vma links for remaining mapped pages in old vma.

 mm/mremap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index f554320281cc..ec6c63e3ea66 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -595,6 +595,14 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 		/* We always clear VM_LOCKED[ONFAULT] on the old vma */
 		vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
 
+		/*
+		 * anon_vma links of the old vma is no longer needed after its page
+		 * table has been moved.
+		 */
+		if (new_vma != vma && vma->vm_start == old_addr &&
+			vma->vm_end == (old_addr + old_len))
+			unlink_anon_vmas(vma);
+
 		/* Because we won't unmap we don't need to touch locked_vm */
 		return new_addr;
 	}
-- 
2.18.4