Subject: track the root (oldest) anon_vma
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@xxxxxxxxxx>
---
include/linux/rmap.h | 1 +
mm/rmap.c | 20 +++++++++++++++++---
2 files changed, 18 insertions(+), 3 deletions(-)
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 72ecd87..457ae1e 100644
--- a/include/linux/rmap.h
+++ b/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 */
#ifdef CONFIG_KSM
atomic_t ksm_refcount;
#endif
diff --git a/mm/rmap.c b/mm/rmap.c
index 6102f77..e34cb56 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -132,6 +132,11 @@ int anon_vma_prepare(struct vm_area_struct *vma)
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);
@@ -203,7 +208,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
*/
int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
{
- struct anon_vma_chain *avc;
+ struct anon_vma_chain *avc, *root_avc;
struct anon_vma *anon_vma;
/* Don't bother if the parent process has no anon_vma here. */
@@ -224,9 +229,18 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
avc = anon_vma_chain_alloc();
if (!avc)
goto out_error_free_anon_vma;
- anon_vma_chain_link(vma, avc, anon_vma);
+
+ /*
+ * Get the root anon_vma on the list by depending on the ordering
+ * of the same_vma list setup by previous invocations of anon_vma_fork.
+ * The root anon_vma will always be referenced by the last item
+ * in the anon_vma_chain list.
+ */
+ root_avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
+ anon_vma->root = root_avc->anon_vma;
/* 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 +275,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
{
struct anon_vma_chain *avc, *next;
- /* Unlink each anon_vma chained to the VMA. */
+ /* Unlink each anon_vma chained to the VMA, from newest to oldest. */
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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