[082/173] mm: rmap: explicitly reset vma->anon_vma in unlink_anon_vmas()

Commit Message

Andrew Morton Feb. 24, 2021, 8:04 p.m. UTC

From: Li Xinhai <[e-mail]>
Subject: 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/[e-mail]
Signed-off-by: Li Xinhai <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Brian Geffon <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Lokesh Gidra <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/rmap.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Patch

--- a/mm/rmap.c~mm-rmap-explicitly-reset-vma-anon_vma-in-unlink_anon_vmas
+++ a/mm/rmap.c
@@ -413,8 +413,15 @@  void unlink_anon_vmas(struct vm_area_str
 		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);
 
 	/*