On Fri,  2 Apr 2010 17:02:42 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

quoted text
> This patch is the core of a mechanism which compacts memory in a zone by
> relocating movable pages towards the end of the zone.
> 
> A single compaction run involves a migration scanner and a free scanner.
> Both scanners operate on pageblock-sized areas in the zone. The migration
> scanner starts at the bottom of the zone and searches for all movable pages
> within each area, isolating them onto a private list called migratelist.
> The free scanner starts at the top of the zone and searches for suitable
> areas and consumes the free pages within making them available for the
> migration scanner. The pages isolated for migration are then migrated to
> the newly isolated free pages.
> 
>
> ...
>
> --- /dev/null
> +++ b/include/linux/compaction.h
> @@ -0,0 +1,9 @@
> +#ifndef _LINUX_COMPACTION_H
> +#define _LINUX_COMPACTION_H
> +
> +/* Return values for compact_zone() */
> +#define COMPACT_INCOMPLETE	0
> +#define COMPACT_PARTIAL		1
> +#define COMPACT_COMPLETE	2

Confused.  "incomplete" and "partial" are synonyms.  Please fully
document these here.

quoted text
> +#endif /* _LINUX_COMPACTION_H */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f3b473a..f920815 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -335,6 +335,7 @@ void put_page(struct page *page);
>  void put_pages_list(struct list_head *pages);
>  
>  void split_page(struct page *page, unsigned int order);
> +int split_free_page(struct page *page);
>  
>  /*
>   * Compound pages have a destructor function.  Provide a
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 986b12d..cf8bba7 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -151,6 +151,7 @@ enum {
>  };
>  
>  #define SWAP_CLUSTER_MAX 32
> +#define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX

Why?  What are the implications of this decision?  How was it arrived
at?  What might one expect if one were to alter COMPACT_CLUSTER_MAX?

quoted text
>  #define SWAP_MAP_MAX	0x3e	/* Max duplication count, in first swap_map */
>  #define SWAP_MAP_BAD	0x3f	/* Note pageblock is bad, in first swap_map */
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 117f0dd..56e4b44 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -43,6 +43,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		KSWAPD_LOW_WMARK_HIT_QUICKLY, KSWAPD_HIGH_WMARK_HIT_QUICKLY,
>  		KSWAPD_SKIP_CONGESTION_WAIT,
>  		PAGEOUTRUN, ALLOCSTALL, PGROTATED,
> +		COMPACTBLOCKS, COMPACTPAGES, COMPACTPAGEFAILED,
>  #ifdef CONFIG_HUGETLB_PAGE
>  		HTLB_BUDDY_PGALLOC, HTLB_BUDDY_PGALLOC_FAIL,
>  #endif
> diff --git a/mm/Makefile b/mm/Makefile
> index 7a68d2a..ccb1f72 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
>
> ...
>
> +static int release_freepages(struct list_head *freelist)
> +{
> +	struct page *page, *next;
> +	int count = 0;
> +
> +	list_for_each_entry_safe(page, next, freelist, lru) {
> +		list_del(&page->lru);
> +		__free_page(page);
> +		count++;
> +	}
> +
> +	return count;
> +}

I'm kinda surprised that we don't already have a function to do this.

An `unsigned' return value would make more sense.  Perhaps even
`unsigned long', unless there's something else here which would prevent
that absurd corner-case.

quoted text
> +/* Isolate free pages onto a private freelist. Must hold zone->lock */
> +static int isolate_freepages_block(struct zone *zone,
> +				unsigned long blockpfn,
> +				struct list_head *freelist)
> +{
> +	unsigned long zone_end_pfn, end_pfn;
> +	int total_isolated = 0;
> +
> +	/* Get the last PFN we should scan for free pages at */
> +	zone_end_pfn = zone->zone_start_pfn + zone->spanned_pages;
> +	end_pfn = blockpfn + pageblock_nr_pages;
> +	if (end_pfn > zone_end_pfn)
> +		end_pfn = zone_end_pfn;

	end_pfn = min(blockpfn + pageblock_nr_pages, zone_end_pfn);

I find that easier to follow, dunno how others feel.

quoted text
> +	/* Isolate free pages. This assumes the block is valid */

What does "This assumes the block is valid" mean?  The code checks
pfn_valid_within()..

quoted text
> +	for (; blockpfn < end_pfn; blockpfn++) {
> +		struct page *page;
> +		int isolated, i;
> +
> +		if (!pfn_valid_within(blockpfn))
> +			continue;
> +
> +		page = pfn_to_page(blockpfn);

hm.  pfn_to_page() isn't exactly cheap in some memory models.  I wonder
if there was some partial result we could have locally cached across
the entire loop.

quoted text
> +		if (!PageBuddy(page))
> +			continue;
> +
> +		/* Found a free page, break it into order-0 pages */
> +		isolated = split_free_page(page);
> +		total_isolated += isolated;
> +		for (i = 0; i < isolated; i++) {
> +			list_add(&page->lru, freelist);
> +			page++;
> +		}
> +
> +		/* If a page was split, advance to the end of it */
> +		if (isolated)
> +			blockpfn += isolated - 1;
> +	}

Strange.  Having just busted a pageblock_order-sized higher-order page
into order-0 pages, the loop goes on and inspects the remaining
(1-2^pageblock_order) pages, presumably to no effect.  Perhaps

	for (; blockpfn < end_pfn; blockpfn++) {

should be

	for (; blockpfn < end_pfn; blockpfn += pageblock_nr_pages) {

or somesuch.

btw, is the whole pageblock_order thing as sucky as it seems?  If I
want my VM to be oriented to making order-4-skb-allocations work, I
need to tune it that way, to coopt something the hugepage fetishists
added?  What if I need order-4 skb's _and_ hugepages?

quoted text
> +	return total_isolated;
> +}
> +
> +/* Returns 1 if the page is within a block suitable for migration to */
> +static int suitable_migration_target(struct page *page)

`bool'?

quoted text
> +{
> +
> +	int migratetype = get_pageblock_migratetype(page);
> +
> +	/* Don't interfere with memory hot-remove or the min_free_kbytes blocks */
> +	if (migratetype == MIGRATE_ISOLATE || migratetype == MIGRATE_RESERVE)
> +		return 0;
> +
> +	/* If the page is a large free page, then allow migration */
> +	if (PageBuddy(page) && page_order(page) >= pageblock_order)
> +		return 1;
> +
> +	/* If the block is MIGRATE_MOVABLE, allow migration */
> +	if (migratetype == MIGRATE_MOVABLE)
> +		return 1;
> +
> +	/* Otherwise skip the block */
> +	return 0;
> +}
> +
> +/*
> + * Based on information in the current compact_control, find blocks
> + * suitable for isolating free pages from

"and then isolate them"?

quoted text
> + */
> +static void isolate_freepages(struct zone *zone,
> +				struct compact_control *cc)
> +{
> +	struct page *page;
> +	unsigned long high_pfn, low_pfn, pfn;
> +	unsigned long flags;
> +	int nr_freepages = cc->nr_freepages;
> +	struct list_head *freelist = &cc->freepages;
> +
> +	pfn = cc->free_pfn;
> +	low_pfn = cc->migrate_pfn + pageblock_nr_pages;
> +	high_pfn = low_pfn;
> +
> +	/*
> +	 * Isolate free pages until enough are available to migrate the
> +	 * pages on cc->migratepages. We stop searching if the migrate
> +	 * and free page scanners meet or enough free pages are isolated.
> +	 */
> +	spin_lock_irqsave(&zone->lock, flags);
> +	for (; pfn > low_pfn && cc->nr_migratepages > nr_freepages;
> +					pfn -= pageblock_nr_pages) {
> +		int isolated;
> +
> +		if (!pfn_valid(pfn))
> +			continue;
> +
> +		/* 
> +		 * Check for overlapping nodes/zones. It's possible on some
> +		 * configurations to have a setup like
> +		 * node0 node1 node0
> +		 * i.e. it's possible that all pages within a zones range of
> +		 * pages do not belong to a single zone.
> +		 */
> +		page = pfn_to_page(pfn);
> +		if (page_zone(page) != zone)
> +			continue;

Well.  This code checks each pfn it touches, but
isolate_freepages_block() doesn't do this - isolate_freepages_block()
happily blunders across a contiguous span of pageframes, assuming that
all those pages are valid, and within the same zone.

quoted text
> +		/* Check the block is suitable for migration */
> +		if (!suitable_migration_target(page))
> +			continue;
> +
> +		/* Found a block suitable for isolating free pages from */
> +		isolated = isolate_freepages_block(zone, pfn, freelist);
> +		nr_freepages += isolated;
> +
> +		/*
> +		 * Record the highest PFN we isolated pages from. When next
> +		 * looking for free pages, the search will restart here as
> +		 * page migration may have returned some pages to the allocator
> +		 */
> +		if (isolated)
> +			high_pfn = max(high_pfn, pfn);
> +	}
> +	spin_unlock_irqrestore(&zone->lock, flags);

For how long can this loop hold of interrupts?

quoted text
> +	cc->free_pfn = high_pfn;
> +	cc->nr_freepages = nr_freepages;
> +}
> +
> +/* Update the number of anon and file isolated pages in the zone */
> +static void acct_isolated(struct zone *zone, struct compact_control *cc)
> +{
> +	struct page *page;
> +	unsigned int count[NR_LRU_LISTS] = { 0, };
> +
> +	list_for_each_entry(page, &cc->migratepages, lru) {
> +		int lru = page_lru_base_type(page);
> +		count[lru]++;
> +	}
> +
> +	cc->nr_anon = count[LRU_ACTIVE_ANON] + count[LRU_INACTIVE_ANON];
> +	cc->nr_file = count[LRU_ACTIVE_FILE] + count[LRU_INACTIVE_FILE];
> +	__mod_zone_page_state(zone, NR_ISOLATED_ANON, cc->nr_anon);
> +	__mod_zone_page_state(zone, NR_ISOLATED_FILE, cc->nr_file);
> +}
> +
> +/* Similar to reclaim, but different enough that they don't share logic */

yeah, but what does it do?

quoted text
> +static int too_many_isolated(struct zone *zone)
> +{
> +
> +	unsigned long inactive, isolated;
> +
> +	inactive = zone_page_state(zone, NR_INACTIVE_FILE) +
> +					zone_page_state(zone, NR_INACTIVE_ANON);
> +	isolated = zone_page_state(zone, NR_ISOLATED_FILE) +
> +					zone_page_state(zone, NR_ISOLATED_ANON);
> +
> +	return isolated > inactive;
> +}
> +
> +/*
> + * Isolate all pages that can be migrated from the block pointed to by
> + * the migrate scanner within compact_control.
> + */
> +static unsigned long isolate_migratepages(struct zone *zone,
> +					struct compact_control *cc)
> +{
> +	unsigned long low_pfn, end_pfn;
> +	struct list_head *migratelist;
> +
> +	low_pfn = cc->migrate_pfn;
> +	migratelist = &cc->migratepages;
> +
> +	/* Do not scan outside zone boundaries */
> +	if (low_pfn < zone->zone_start_pfn)
> +		low_pfn = zone->zone_start_pfn;

Can this happen?

Use max()?

quoted text
> +	/* Setup to scan one block but not past where we are migrating to */

what?

quoted text
> +	end_pfn = ALIGN(low_pfn + pageblock_nr_pages, pageblock_nr_pages);
> +
> +	/* Do not cross the free scanner or scan within a memory hole */
> +	if (end_pfn > cc->free_pfn || !pfn_valid(low_pfn)) {
> +		cc->migrate_pfn = end_pfn;
> +		return 0;
> +	}
> +
> +	/* Do not isolate the world */

Needs (much) more explanation, please.

quoted text
> +	while (unlikely(too_many_isolated(zone))) {
> +		congestion_wait(BLK_RW_ASYNC, HZ/10);

... why did it do this?  Quite a head-scratcher.

quoted text
> +		if (fatal_signal_pending(current))
> +			return 0;
> +	}
> +
> +	/* Time to isolate some pages for migration */
> +	spin_lock_irq(&zone->lru_lock);
> +	for (; low_pfn < end_pfn; low_pfn++) {
> +		struct page *page;
> +		if (!pfn_valid_within(low_pfn))
> +			continue;
> +
> +		/* Get the page and skip if free */
> +		page = pfn_to_page(low_pfn);
> +		if (PageBuddy(page)) {
> +			low_pfn += (1 << page_order(page)) - 1;
> +			continue;
> +		}
> +
> +		/* Try isolate the page */
> +		if (__isolate_lru_page(page, ISOLATE_BOTH, 0) == 0) {
> +			del_page_from_lru_list(zone, page, page_lru(page));
> +			list_add(&page->lru, migratelist);
> +			mem_cgroup_del_lru(page);
> +			cc->nr_migratepages++;
> +		}
> +
> +		/* Avoid isolating too much */
> +		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
> +			break;

This test could/should be moved inside the preceding `if' block.  Or,
better, simply do

		if (__isolate_lru_page(page, ISOLATE_BOTH, 0) != 0)
			continue;	/* comment goes here */

quoted text
> +	}
> +
> +	acct_isolated(zone, cc);
> +
> +	spin_unlock_irq(&zone->lru_lock);
> +	cc->migrate_pfn = low_pfn;
> +
> +	return cc->nr_migratepages;
> +}
> +
> +/*
> + * This is a migrate-callback that "allocates" freepages by taking pages
> + * from the isolated freelists in the block we are migrating to.
> + */
> +static struct page *compaction_alloc(struct page *migratepage,
> +					unsigned long data,
> +					int **result)
> +{
> +	struct compact_control *cc = (struct compact_control *)data;
> +	struct page *freepage;
> +
> +	/* Isolate free pages if necessary */
> +	if (list_empty(&cc->freepages)) {
> +		isolate_freepages(cc->zone, cc);
> +
> +		if (list_empty(&cc->freepages))
> +			return NULL;
> +	}
> +
> +	freepage = list_entry(cc->freepages.next, struct page, lru);
> +	list_del(&freepage->lru);
> +	cc->nr_freepages--;
> +
> +	return freepage;
> +}
> +
> +/*
> + * We cannot control nr_migratepages and nr_freepages fully when migration is
> + * running as migrate_pages() has no knowledge of compact_control. When
> + * migration is complete, we count the number of pages on the lists by hand.
> + */
> +static void update_nr_listpages(struct compact_control *cc)
> +{
> +	int nr_migratepages = 0;
> +	int nr_freepages = 0;
> +	struct page *page;

newline here please.

quoted text
> +	list_for_each_entry(page, &cc->migratepages, lru)
> +		nr_migratepages++;
> +	list_for_each_entry(page, &cc->freepages, lru)
> +		nr_freepages++;
> +
> +	cc->nr_migratepages = nr_migratepages;
> +	cc->nr_freepages = nr_freepages;
> +}
> +
> +static inline int compact_finished(struct zone *zone,
> +						struct compact_control *cc)
> +{
> +	if (fatal_signal_pending(current))
> +		return COMPACT_PARTIAL;

ah-hah!  So maybe we meant COMPACT_INTERRUPTED.

quoted text
> +	/* Compaction run completes if the migrate and free scanner meet */
> +	if (cc->free_pfn <= cc->migrate_pfn)
> +		return COMPACT_COMPLETE;
> +
> +	return COMPACT_INCOMPLETE;
> +}
> +
> +static int compact_zone(struct zone *zone, struct compact_control *cc)
> +{
> +	int ret = COMPACT_INCOMPLETE;
> +
> +	/* Setup to move all movable pages to the end of the zone */
> +	cc->migrate_pfn = zone->zone_start_pfn;
> +	cc->free_pfn = cc->migrate_pfn + zone->spanned_pages;
> +	cc->free_pfn &= ~(pageblock_nr_pages-1);

If zone->spanned_pages is much much larger than zone->present_pages,
this code will suck rather a bit.  Is there a reason why that can never
happen?

quoted text
> +	migrate_prep();
> +
> +	for (; ret == COMPACT_INCOMPLETE; ret = compact_finished(zone, cc)) {

<stares at that for a while>

Perhaps

	while ((ret = compact_finished(zone, cc)) == COMPACT_INCOMPLETE) {

would be clearer.  That would make the definition-site initialisation
of `ret' unneeded too.

quoted text
> +		unsigned long nr_migrate, nr_remaining;

newline please.

quoted text
> +		if (!isolate_migratepages(zone, cc))
> +			continue;

Boy, this looks like an infinite loop waiting to happen.  Are you sure?
Suppose we hit a pageblock-sized string of !pfn_valid() pfn's, for
example.  Worried.

quoted text
> +		nr_migrate = cc->nr_migratepages;
> +		migrate_pages(&cc->migratepages, compaction_alloc,
> +						(unsigned long)cc, 0);
> +		update_nr_listpages(cc);
> +		nr_remaining = cc->nr_migratepages;
> +
> +		count_vm_event(COMPACTBLOCKS);
> +		count_vm_events(COMPACTPAGES, nr_migrate - nr_remaining);
> +		if (nr_remaining)
> +			count_vm_events(COMPACTPAGEFAILED, nr_remaining);
> +
> +		/* Release LRU pages not migrated */
> +		if (!list_empty(&cc->migratepages)) {
> +			putback_lru_pages(&cc->migratepages);
> +			cc->nr_migratepages = 0;
> +		}
> +
> +	}
> +
> +	/* Release free pages and check accounting */
> +	cc->nr_freepages -= release_freepages(&cc->freepages);
> +	VM_BUG_ON(cc->nr_freepages != 0);
> +
> +	return ret;
> +}
> +
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 624cba4..3cf947d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1208,6 +1208,45 @@ void split_page(struct page *page, unsigned int order)
>  }
>  
>  /*
> + * Similar to split_page except the page is already free. As this is only
> + * being used for migration, the migratetype of the block also changes.
> + */
> +int split_free_page(struct page *page)
> +{
> +	unsigned int order;
> +	unsigned long watermark;
> +	struct zone *zone;
> +
> +	BUG_ON(!PageBuddy(page));
> +
> +	zone = page_zone(page);
> +	order = page_order(page);
> +
> +	/* Obey watermarks or the system could deadlock */
> +	watermark = low_wmark_pages(zone) + (1 << order);
> +	if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
> +		return 0;

OK, there is no way in which the code-reader can work out why this is
here.  What deadlock?

quoted text
> +	/* Remove page from free list */
> +	list_del(&page->lru);
> +	zone->free_area[order].nr_free--;
> +	rmv_page_order(page);
> +	__mod_zone_page_state(zone, NR_FREE_PAGES, -(1UL << order));
> +
> +	/* Split into individual pages */
> +	set_page_refcounted(page);
> +	split_page(page, order);
> +
> +	if (order >= pageblock_order - 1) {
> +		struct page *endpage = page + (1 << order) - 1;
> +		for (; page < endpage; page += pageblock_nr_pages)
> +			set_pageblock_migratetype(page, MIGRATE_MOVABLE);
> +	}
> +
> +	return 1 << order;
> +}
> +
> +/*
>   * Really, prep_compound_page() should be called from __rmqueue_bulk().  But
>   * we cheat by calling it from here, in the order > 0 path.  Saves a branch
>   * or two.
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 351e491..3a69b48 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -892,6 +892,11 @@ static const char * const vmstat_text[] = {
>  	"allocstall",
>  
>  	"pgrotated",
> +
> +	"compact_blocks_moved",
> +	"compact_pages_moved",
> +	"compact_pagemigrate_failed",

Should we present these on CONFIG_COMPACTION=n kernels?



Does all this code really need to iterate across individual pfn's like
this?  We can use the buddy structures to go straight to all of a
zone's order-N free pages, can't we?  Wouldn't that save a whole heap
of fruitless linear searching?
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