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

Minchan Kim
2013-06-04T06:59:34Z

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