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

[Minchan Kim <minchan@kernel.org>]

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().

I missed that point.

> 
> '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.
> 

Right.

> 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

                                        maybe you mean
                                        but doing this doesn't really helps
> lock_page() latency.
> 
> Does that make sense?

If so, It does make sense.
Thanks for pointing me out.

> 
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-- 
Kind regards,
Minchan Kim

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