Re: [PATCH 4/4] capture pages freed during direct reclaim for allocation by the reclaimer

Peter Zijlstra
2008-09-04T07:20:18Z

On Wed, 2008-09-03 at 21:53 +0100, Andy Whitcroft wrote:
> [Doh, as pointed out by Christoph the patch was missing from this one...]
>
> When a process enters direct reclaim it will expend effort identifying
> and releasing pages in the hope of obtaining a page. However as these
> pages are released asynchronously there is every possibility that the
> pages will have been consumed by other allocators before the reclaimer
> gets a look in. This is particularly problematic where the reclaimer is
> attempting to allocate a higher order page. It is highly likely that
> a parallel allocation will consume lower order constituent pages as we
> release them preventing them coelescing into the higher order page the
> reclaimer desires.
>
> This patch set attempts to address this for allocations above
> ALLOC_COSTLY_ORDER by temporarily collecting the pages we are releasing
> onto a local free list. Instead of freeing them to the main buddy lists,
> pages are collected and coelesced on this per direct reclaimer free list.
> Pages which are freed by other processes are also considered, where they
> coelesce with a page already under capture they will be moved to the
> capture list. When pressure has been applied to a zone we then consult
> the capture list and if there is an appropriatly sized page available
> it is taken immediatly and the remainder returned to the free pool.
> Capture is only enabled when the reclaimer's allocation order exceeds
> ALLOC_COSTLY_ORDER as free pages below this order should naturally occur
> in large numbers following regular reclaim.
>
> Thanks go to Mel Gorman for numerous discussions during the development
> of this patch and for his repeated reviews.

Whole series looks good, a few comments below.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> Signed-off-by: Andy Whitcroft <apw@shadowen.org>
> ---

> @@ -4815,6 +4900,73 @@ out:
> spin_unlock_irqrestore(&zone->lock, flags);
> }
>
> +#define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))
> +
> +/*
> + * Run through the accumulated list of captured pages and the first
> + * which is big enough to satisfy the original allocation. Free
> + * the remainder of that page and all other pages.
> + */

That sentence looks incomplete, did you intend to write something along
the lines of:

Run through the accumulated list of captures pages and /take/ the first
which is big enough to satisfy the original allocation. Free the
remaining pages.

?

> +struct page *capture_alloc_or_return(struct zone *zone,
> + struct zone *preferred_zone, struct list_head *capture_list,
> + int order, int alloc_flags, gfp_t gfp_mask)
> +{
> + struct page *capture_page = 0;
> + unsigned long flags;
> + int classzone_idx = zone_idx(preferred_zone);
> +
> + spin_lock_irqsave(&zone->lock, flags);
> +
> + while (!list_empty(capture_list)) {
> + struct page *page;
> + int pg_order;
> +
> + page = lru_to_page(capture_list);
> + list_del(&page->lru);
> + pg_order = page_order(page);
> +
> + /*
> + * Clear out our buddy size and list information before
> + * releasing or allocating the page.
> + */
> + rmv_page_order(page);
> + page->buddy_free = 0;
> + ClearPageBuddyCapture(page);
> +
> + if (!capture_page && pg_order >= order) {
> + __carve_off(page, pg_order, order);
> + capture_page = page;
> + } else
> + __free_one_page(page, zone, pg_order);
> + }
> +
> + /*
> + * Ensure that this capture would not violate the watermarks.
> + * Subtle, we actually already have the page outside the watermarks
> + * so check if we can allocate an order 0 page.
> + */
> + if (capture_page &&
> + (!zone_cpuset_permits(zone, alloc_flags, gfp_mask) ||
> + !zone_watermark_permits(zone, 0, classzone_idx,
> + alloc_flags, gfp_mask))) {
> + __free_one_page(capture_page, zone, order);
> + capture_page = NULL;
> + }

This makes me a little sad - we got a high order page and give it away
again...

Can we start another round of direct reclaim with a lower order to try
and increase the watermarks while we hold on to this large order page?

> + if (capture_page)
> + __count_zone_vm_events(PGALLOC, zone, 1 << order);
> +
> + zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);
> + zone->pages_scanned = 0;
> +
> + spin_unlock_irqrestore(&zone->lock, flags);
> +
> + if (capture_page)
> + prep_new_page(capture_page, order, gfp_mask);
> +
> + return capture_page;
> +}
> +
> #ifdef CONFIG_MEMORY_HOTREMOVE
> /*
> * All pages in the range must be isolated before calling this.