Re: [v5][PATCH 5/6] mm: vmscan: batch shrink_page_list() locking operations

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Mon, Jun 03, 2013 at 11:10:02PM -0700, Dave Hansen wrote:
>On 06/03/2013 10:01 PM, Minchan Kim wrote:
>>> > +static int __remove_mapping_batch(struct list_head *remove_list,
>>> > +				  struct list_head *ret_pages,
>>> > +				  struct list_head *free_pages)
>>> > +{
>>> > +	int nr_reclaimed = 0;
>>> > +	struct address_space *mapping;
>>> > +	struct page *page;
>>> > +	LIST_HEAD(need_free_mapping);
>>> > +
>>> > +	while (!list_empty(remove_list)) {
>...
>>> > +		if (!__remove_mapping(mapping, page)) {
>>> > +			unlock_page(page);
>>> > +			list_add(&page->lru, ret_pages);
>>> > +			continue;
>>> > +		}
>>> > +		list_add(&page->lru, &need_free_mapping);
>...
>> +	spin_unlock_irq(&mapping->tree_lock);
>> +	while (!list_empty(&need_free_mapping)) {...
>> +		list_move(&page->list, free_pages);
>> +		mapping_release_page(mapping, page);
>> +	}
>> Why do we need new lru list instead of using @free_pages?
>
>I actually tried using @free_pages at first.  The problem is that we
>need to call mapping_release_page() without the radix tree lock held so
>we can not do it in the first while() loop.
>
>'free_pages' is a list created up in shrink_page_list().  There can be
>several calls to __remove_mapping_batch() for each call to
>shrink_page_list().
>
>'need_free_mapping' lets us temporarily differentiate the pages that we
>need to call mapping_release_page()/unlock_page() on versus the ones on
>'free_pages' which have already had that done.
>
>We could theoretically delay _all_ of the
>release_mapping_page()/unlock_page() operations until the _entire_
>shrink_page_list() operation is done, but doing this really helps with
>lock_page() latency.
>
>Does that make sense?

Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>

>
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