Re: [PATCH] mmu notifiers #v7

Christoph Lameter
2008-02-28T23:05:50Z

On Wed, 27 Feb 2008, Andrea Arcangeli wrote:

> +struct mmu_notifier_head {
> + struct hlist_head head;
> + spinlock_t lock;
> +};

Still think that the lock here is not of too much use and can be easily
replaced by mmap_sem.

> +#define mmu_notifier(function, mm, args...) \
> + do { \
> + struct mmu_notifier *__mn; \
> + struct hlist_node *__n; \
> + \
> + if (unlikely(!hlist_empty(&(mm)->mmu_notifier.head))) { \
> + rcu_read_lock(); \
> + hlist_for_each_entry_rcu(__mn, __n, \
> + &(mm)->mmu_notifier.head, \
> + hlist) \
> + if (__mn->ops->function) \
> + __mn->ops->function(__mn, \
> + mm, \
> + args); \
> + rcu_read_unlock(); \
> + } \
> + } while (0)

Andrew recomended local variables for parameters used multile times. This
means the mm parameter here.

> +/*
> + * Notifiers that use the parameters that they were passed so that the
> + * compiler does not complain about unused variables but does proper
> + * parameter checks even if !CONFIG_MMU_NOTIFIER.
> + * Macros generate no code.
> + */
> +#define mmu_notifier(function, mm, args...) \
> + do { \
> + if (0) { \
> + struct mmu_notifier *__mn; \
> + \
> + __mn = (struct mmu_notifier *)(0x00ff); \
> + __mn->ops->function(__mn, mm, args); \
> + }; \
> + } while (0)

Note also Andrew's comments on the use of 0x00ff...

> +/*
> + * No synchronization. This function can only be called when only a single
> + * process remains that performs teardown.
> + */
> +void mmu_notifier_release(struct mm_struct *mm)
> +{
> + struct mmu_notifier *mn;
> + struct hlist_node *n, *tmp;
> +
> + if (unlikely(!hlist_empty(&mm->mmu_notifier.head))) {
> + hlist_for_each_entry_safe(mn, n, tmp,
> + &mm->mmu_notifier.head, hlist) {
> + hlist_del(&mn->hlist);
> + if (mn->ops->release)
> + mn->ops->release(mn, mm);
> + }
> + }
> +}

One could avoid a hlist_for_each_entry_safe here by simply always deleting
the first object.

Also re the _notify variants: The binding to pte_clear_flush_young etc
will become a problem for notifiers that want to sleep because
pte_clear_flush is usually called with the pte lock held. See f.e.
try_to_unmap_one, page_mkclean_one etc.

It would be better if the notifier calls could be moved outside of the
pte lock.