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


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