On Fri, 2 Apr 2010 17:02:35 +0100
Mel Gorman <mel@xxxxxxxxx> wrote:
> rmap_walk_anon() does not use page_lock_anon_vma() for looking up and
> locking an anon_vma and it does not appear to have sufficient locking to
> ensure the anon_vma does not disappear from under it.
>
> This patch copies an approach used by KSM to take a reference on the
> anon_vma while pages are being migrated. This should prevent rmap_walk()
> running into nasty surprises later because anon_vma has been freed.
>
The code didn't exactly bend over backwards making itself easy for
others to understand...
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index d25bd22..567d43f 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -29,6 +29,9 @@ struct anon_vma {
> #ifdef CONFIG_KSM
> atomic_t ksm_refcount;
> #endif
> +#ifdef CONFIG_MIGRATION
> + atomic_t migrate_refcount;
> +#endif
Some documentation here describing the need for this thing and its
runtime semantics would be appropriate.
> /*
> * NOTE: the LSB of the head.next is set by
> * mm_take_all_locks() _after_ taking the above lock. So the
> @@ -81,6 +84,26 @@ static inline int ksm_refcount(struct anon_vma *anon_vma)
> return 0;
> }
> #endif /* CONFIG_KSM */
> +#ifdef CONFIG_MIGRATION
> +static inline void migrate_refcount_init(struct anon_vma *anon_vma)
> +{
> + atomic_set(&anon_vma->migrate_refcount, 0);
> +}
> +
> +static inline int migrate_refcount(struct anon_vma *anon_vma)
> +{
> + return atomic_read(&anon_vma->migrate_refcount);
> +}
> +#else
> +static inline void migrate_refcount_init(struct anon_vma *anon_vma)
> +{
> +}
> +
> +static inline int migrate_refcount(struct anon_vma *anon_vma)
> +{
> + return 0;
> +}
> +#endif /* CONFIG_MIGRATE */
>
> static inline struct anon_vma *page_anon_vma(struct page *page)
> {
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6903abf..06e6316 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -542,6 +542,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> int rcu_locked = 0;
> int charge = 0;
> struct mem_cgroup *mem = NULL;
> + struct anon_vma *anon_vma = NULL;
>
> if (!newpage)
> return -ENOMEM;
> @@ -598,6 +599,8 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> if (PageAnon(page)) {
> rcu_read_lock();
> rcu_locked = 1;
> + anon_vma = page_anon_vma(page);
> + atomic_inc(&anon_vma->migrate_refcount);
So no helper function for this. I guess a grep for `migrate_refcount'
will find it OK.
Can this count ever have a value > 1? I guess so..
> }
>
> /*
> @@ -637,6 +640,15 @@ skip_unmap:
> if (rc)
> remove_migration_ptes(page, page);
> rcu_unlock:
> +
> + /* Drop an anon_vma reference if we took one */
> + if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
> + int empty = list_empty(&anon_vma->head);
> + spin_unlock(&anon_vma->lock);
> + if (empty)
> + anon_vma_free(anon_vma);
> + }
> +
So... Why shouldn't this be testing ksm_refcount() too?
Can we consolidate ksm_refcount and migrate_refcount into, err, `refcount'?
> if (rcu_locked)
> rcu_read_unlock();
> uncharge:
> diff --git a/mm/rmap.c b/mm/rmap.c
> index fcd593c..578d0fe 100644
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