Re: [patch 3/6] mm: speculative get page

[Christoph Lameter <clameter@sgi.com>]

On Sun, 11 Nov 2007, Nick Piggin wrote:

> +static inline int page_freeze_refs(struct page *page, int count)
> +{
> +	return likely(atomic_cmpxchg(&page->_count, count, 0) == count);
> +}
> +
> +static inline void page_unfreeze_refs(struct page *page, int count)
> +{
> +	VM_BUG_ON(page_count(page) != 0);
> +	VM_BUG_ON(count == 0);
> +
> +	atomic_set(&page->_count, count);
> +}

Good idea. That avoids another page bit.

> Index: linux-2.6/mm/migrate.c
> ===================================================================
> --- linux-2.6.orig/mm/migrate.c
> +++ linux-2.6/mm/migrate.c
> @@ -294,6 +294,7 @@ out:
>  static int migrate_page_move_mapping(struct address_space *mapping,
>  		struct page *newpage, struct page *page)
>  {
> +	int expected_count;
>  	void **pslot;
>  
>  	if (!mapping) {
> @@ -308,12 +309,18 @@ static int migrate_page_move_mapping(str
>  	pslot = radix_tree_lookup_slot(&mapping->page_tree,
>   					page_index(page));
>  
> -	if (page_count(page) != 2 + !!PagePrivate(page) ||
> +	expected_count = 2 + !!PagePrivate(page);
> +	if (page_count(page) != expected_count ||
>  			(struct page *)radix_tree_deref_slot(pslot) != page) {
>  		write_unlock_irq(&mapping->tree_lock);
>  		return -EAGAIN;
>  	}
>  
> +	if (!page_freeze_refs(page, expected_count))
> +		write_unlock_irq(&mapping->tree_lock);
> +		return -EAGAIN;
> +	}
> +

Looks okay but I think you could remove the earlier performance check. We 
already modified the page struct by obtaining the page lock so we hold it 
exclusively. And the failure rate here is typicalyvery low.

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