Re: [PATCH v3 2/3] Call the filesystem back whenever a page is removed from the page cache

[Hugh Dickins <hughd@google.com>]

On Wed, 1 Dec 2010, Trond Myklebust wrote:
> 
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -108,6 +108,10 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
>  	clear_page_mlock(page);
>  	remove_from_page_cache(page);
>  	ClearPageMappedToDisk(page);
> +
> +	if (mapping->a_ops->freepage)
> +		mapping->a_ops->freepage(page);
> +
>  	page_cache_release(page);	/* pagecache ref */
>  	return 0;
>  }

I think Linus recommended that one be done in remove_from_page_cache()
to catch all instances: did that get overruled later for some reason?

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -454,6 +454,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
>  	BUG_ON(!PageLocked(page));
>  	BUG_ON(mapping != page_mapping(page));
>  
> +	preempt_disable();
>  	spin_lock_irq(&mapping->tree_lock);
>  	/*
>  	 * The non racy check for a busy page.
> @@ -492,10 +493,19 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
>  		swp_entry_t swap = { .val = page_private(page) };
>  		__delete_from_swap_cache(page);
>  		spin_unlock_irq(&mapping->tree_lock);
> +		preempt_enable();
>  		swapcache_free(swap, page);
>  	} else {
> +		void (*freepage)(struct page *);
> +
> +		freepage = mapping->a_ops->freepage;
> +
>  		__remove_from_page_cache(page);
>  		spin_unlock_irq(&mapping->tree_lock);
> +		if (freepage != NULL)
> +			freepage(page);
> +		preempt_enable();
> +
>  		mem_cgroup_uncharge_cache_page(page);
>  	}
>  
> @@ -503,6 +513,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
>  
>  cannot_free:
>  	spin_unlock_irq(&mapping->tree_lock);
> +	preempt_enable();
>  	return 0;
>  }

I took his "stop_machine()" explanation ("an idle period for everything.
And just a preemption reschedule isn't enough for that") to imply that
there's no need for your preempt_disable/preempt_enable there: they
don't add anything to the module unload case, and they don't help the
spin_unlock_irq issue (and you're already being rightly careful to note
freepage in advance).

But maybe I misunderstood.

Hugh