mm: rmap: explicitly reset vma->anon_vma in unlink_anon_vmas()

In case the vma will continue to be used after unlink its relevant
anon_vma, we need to reset the vma->anon_vma pointer to NULL.  So, later
when fault happen within this vma again, a new anon_vma will be prepared.

By this way, the vma will only be checked for reverse mapping of pages
which been fault in after the unlink_anon_vmas call.

Currently, the mremap with MREMAP_DONTUNMAP scenario will continue use the
vma after moved its page table entries to a new vma.  For other scenarios,
the vma itself will be freed after call unlink_anon_vmas.

Link: https://lkml.kernel.org/r/20210119075126.3513154-1-lixinhai.lxh@gmail.com
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 5ebf16f..e26ae11 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -413,8 +413,15 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
                list_del(&avc->same_vma);
                anon_vma_chain_free(avc);
        }
-       if (vma->anon_vma)
+       if (vma->anon_vma) {
                vma->anon_vma->degree--;
+
+               /*
+                * vma would still be needed after unlink, and anon_vma will be prepared
+                * when handle fault.
+                */
+               vma->anon_vma = NULL;
+       }
        unlock_anon_vma_root(root);
 
        /*