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Subject[PATCH 05/28] track the root (oldest) anon_vma
From: Rik van Riel <riel@redhat.com>

Track the root (oldest) anon_vma in each anon_vma tree. Because we only
take the lock on the root anon_vma, we cannot use the lock on higher-up
anon_vmas to lock anything. This makes it impossible to do an indirect
lookup of the root anon_vma, since the data structures could go away from
under us.

However, a direct pointer is safe because the root anon_vma is always the
last one that gets freed on munmap or exit, by virtue of the same_vma list
order and unlink_anon_vmas walking the list forward.

Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100526154010.3904df5c@annuminas.surriel.com>
---
include/linux/rmap.h | 1 +
mm/rmap.c | 18 ++++++++++++++++--
2 files changed, 17 insertions(+), 2 deletions(-)
Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h
+++ linux-2.6/include/linux/rmap.h
@@ -26,6 +26,7 @@
*/
struct anon_vma {
spinlock_t lock; /* Serialize access to vma list */
+ struct anon_vma *root; /* Root of this anon_vma tree */
#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)

/*
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -132,6 +132,11 @@ int anon_vma_prepare(struct vm_area_stru
if (unlikely(!anon_vma))
goto out_enomem_free_avc;
allocated = anon_vma;
+ /*
+ * This VMA had no anon_vma yet. This anon_vma is
+ * the root of any anon_vma tree that might form.
+ */
+ anon_vma->root = anon_vma;
}

anon_vma_lock(anon_vma);
@@ -224,9 +229,15 @@ int anon_vma_fork(struct vm_area_struct
avc = anon_vma_chain_alloc();
if (!avc)
goto out_error_free_anon_vma;
- anon_vma_chain_link(vma, avc, anon_vma);
+
+ /*
+ * The root anon_vm's spinlock is the lock actually used when we
+ * lock any of the anon_vmas in this anon_vma tree.
+ */
+ anon_vma->root = pvma->anon_vma->root;
/* Mark this anon_vma as the one where our new (COWed) pages go. */
vma->anon_vma = anon_vma;
+ anon_vma_chain_link(vma, avc, anon_vma);

return 0;

@@ -261,7 +272,10 @@ void unlink_anon_vmas(struct vm_area_str
{
struct anon_vma_chain *avc, *next;

- /* Unlink each anon_vma chained to the VMA. */
+ /*
+ * Unlink each anon_vma chained to the VMA. This list is ordered
+ * from newest to oldest, ensuring the root anon_vma gets freed last.
+ */
list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
anon_vma_unlink(avc);
list_del(&avc->same_vma);



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