On Thu, 2011-06-16 at 20:58 -0700, Linus Torvalds wrote:
> Ok, I'm still thinking. I have an approach that I think will handle it
> fairly cleanly, but that involves walking the same_vma list twice:
> once to actually unlink the anon_vma's under the lock, and then a
> second pass that does the rest. It should work.
Something like so? Compiles and runs the benchmark in question.
---
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -324,36 +324,52 @@ int anon_vma_fork(struct vm_area_struct
return -ENOMEM;
}
-static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
+static int anon_vma_unlink(struct anon_vma_chain *anon_vma_chain, struct anon_vma *anon_vma)
{
- struct anon_vma *anon_vma = anon_vma_chain->anon_vma;
- int empty;
-
- /* If anon_vma_fork fails, we can get an empty anon_vma_chain. */
- if (!anon_vma)
- return;
-
- anon_vma_lock(anon_vma);
list_del(&anon_vma_chain->same_anon_vma);
/* We must garbage collect the anon_vma if it's empty */
- empty = list_empty(&anon_vma->head);
- anon_vma_unlock(anon_vma);
+ if (list_empty(&anon_vma->head))
+ return 1;
- if (empty)
- put_anon_vma(anon_vma);
+ return 0;
}
void unlink_anon_vmas(struct vm_area_struct *vma)
{
struct anon_vma_chain *avc, *next;
+ struct anon_vma *root = NULL;
/*
* 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);
+ struct anon_vma *anon_vma = avc->anon_vma;
+
+ /* If anon_vma_fork fails, we can get an empty anon_vma_chain. */
+ if (anon_vma) {
+ root = lock_anon_vma_root(root, anon_vma);
+ /* Leave empty anon_vmas on the list. */
+ if (anon_vma_unlink(avc, anon_vma))
+ continue;
+ }
+ list_del(&avc->same_vma);
+ anon_vma_chain_free(avc);
+ }
+ unlock_anon_vma_root(root);
+
+ /*
+ * Iterate the list once more, it now only contains empty and unlinked
+ * anon_vmas, destroy them. Could not do before due to anon_vma_free()
+ * needing to acquire the anon_vma->root->mutex.
+ */
+ list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
+ struct anon_vma *anon_vma = avc->anon_vma;
+
+ if (anon_vma)
+ put_anon_vma(anon_vma);
+
list_del(&avc->same_vma);
anon_vma_chain_free(avc);
}
--
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