Date:	Mon, 28 Jul 2014 17:29:40 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH v5 05/14] mm, compaction: move pageblock checks up from
 isolate_migratepages_range()

On Mon, 28 Jul 2014, Vlastimil Babka wrote:

> isolate_migratepages_range() is the main function of the compaction scanner,
> called either on a single pageblock by isolate_migratepages() during regular
> compaction, or on an arbitrary range by CMA's __alloc_contig_migrate_range().
> It currently perfoms two pageblock-wide compaction suitability checks, and
> because of the CMA callpath, it tracks if it crossed a pageblock boundary in
> order to repeat those checks.
> 
> However, closer inspection shows that those checks are always true for CMA:
> - isolation_suitable() is true because CMA sets cc->ignore_skip_hint to true
> - migrate_async_suitable() check is skipped because CMA uses sync compaction
> 
> We can therefore move the compaction-specific checks to isolate_migratepages()
> and simplify isolate_migratepages_range(). Furthermore, we can mimic the
> freepage scanner family of functions, which has isolate_freepages_block()
> function called both by compaction from isolate_freepages() and by CMA from
> isolate_freepages_range(), where each use-case adds own specific glue code.
> This allows further code simplification.
> 
> Therefore, we rename isolate_migratepages_range() to isolate_freepages_block()

s/isolate_freepages_block/isolate_migratepages_block/

I read your commit description before looking at the patch and was very 
nervous about the direction you were going if that was true :)  I'm 
relieved to see it was just a typo.

> and limit its functionality to a single pageblock (or its subset). For CMA,
> a new different isolate_migratepages_range() is created as a CMA-specific
> wrapper for the _block() function. The checks specific to compaction are moved
> to isolate_migratepages(). As part of the unification of these two families of
> functions, we remove the redundant zone parameter where applicable, since zone
> pointer is already passed in cc->zone.
> 
> Furthermore, going back to compact_zone() and compact_finished() when pageblock
> is found unsuitable (now by isolate_migratepages()) is wasteful - the checks
> are meant to skip pageblocks quickly. The patch therefore also introduces a
> simple loop into isolate_migratepages() so that it does not return immediately
> on failed pageblock checks, but keeps going until isolate_migratepages_range()
> gets called once. Similarily to isolate_freepages(), the function periodically
> checks if it needs to reschedule or abort async compaction.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Cc: Minchan Kim <minchan@...nel.org>
> Acked-by: Mel Gorman <mgorman@...e.de>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Michal Nazarewicz <mina86@...a86.com>
> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: David Rientjes <rientjes@...gle.com>
> ---
>  mm/compaction.c | 234 +++++++++++++++++++++++++++++++++-----------------------
>  mm/internal.h   |   4 +-
>  mm/page_alloc.c |   3 +-
>  3 files changed, 140 insertions(+), 101 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 0a871e5..bac6e37 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -132,7 +132,7 @@ void reset_isolation_suitable(pg_data_t *pgdat)
>   */
>  static void update_pageblock_skip(struct compact_control *cc,
>  			struct page *page, unsigned long nr_isolated,
> -			bool set_unsuitable, bool migrate_scanner)
> +			bool migrate_scanner)
>  {
>  	struct zone *zone = cc->zone;
>  	unsigned long pfn;
> @@ -146,12 +146,7 @@ static void update_pageblock_skip(struct compact_control *cc,
>  	if (nr_isolated)
>  		return;
>  
> -	/*
> -	 * Only skip pageblocks when all forms of compaction will be known to
> -	 * fail in the near future.
> -	 */
> -	if (set_unsuitable)
> -		set_pageblock_skip(page);
> +	set_pageblock_skip(page);
>  
>  	pfn = page_to_pfn(page);
>  
> @@ -180,7 +175,7 @@ static inline bool isolation_suitable(struct compact_control *cc,
>  
>  static void update_pageblock_skip(struct compact_control *cc,
>  			struct page *page, unsigned long nr_isolated,
> -			bool set_unsuitable, bool migrate_scanner)
> +			bool migrate_scanner)
>  {
>  }
>  #endif /* CONFIG_COMPACTION */
> @@ -345,8 +340,7 @@ isolate_fail:
>  
>  	/* Update the pageblock-skip if the whole pageblock was scanned */
>  	if (blockpfn == end_pfn)
> -		update_pageblock_skip(cc, valid_page, total_isolated, true,
> -				      false);
> +		update_pageblock_skip(cc, valid_page, total_isolated, false);
>  
>  	count_compact_events(COMPACTFREE_SCANNED, nr_scanned);
>  	if (total_isolated)
> @@ -451,40 +445,34 @@ static bool too_many_isolated(struct zone *zone)
>  }
>  
>  /**
> - * isolate_migratepages_range() - isolate all migrate-able pages in range.
> - * @zone:	Zone pages are in.
> + * isolate_migratepages_block() - isolate all migrate-able pages within
> + *				  a single pageblock
>   * @cc:		Compaction control structure.
> - * @low_pfn:	The first PFN of the range.
> - * @end_pfn:	The one-past-the-last PFN of the range.
> - * @unevictable: true if it allows to isolate unevictable pages
> + * @low_pfn:	The first PFN to isolate
> + * @end_pfn:	The one-past-the-last PFN to isolate, within same pageblock
> + * @isolate_mode: Isolation mode to be used.
>   *
>   * Isolate all pages that can be migrated from the range specified by
> - * [low_pfn, end_pfn).  Returns zero if there is a fatal signal
> - * pending), otherwise PFN of the first page that was not scanned
> - * (which may be both less, equal to or more then end_pfn).
> - *
> - * Assumes that cc->migratepages is empty and cc->nr_migratepages is
> - * zero.
> + * [low_pfn, end_pfn). The range is expected to be within same pageblock.
> + * Returns zero if there is a fatal signal pending, otherwise PFN of the
> + * first page that was not scanned (which may be both less, equal to or more
> + * than end_pfn).
>   *
> - * Apart from cc->migratepages and cc->nr_migratetypes this function
> - * does not modify any cc's fields, in particular it does not modify
> - * (or read for that matter) cc->migrate_pfn.
> + * The pages are isolated on cc->migratepages list (not required to be empty),
> + * and cc->nr_migratepages is updated accordingly. The cc->migrate_pfn field
> + * is neither read nor updated.
>   */
> -unsigned long
> -isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> -		unsigned long low_pfn, unsigned long end_pfn, bool unevictable)
> +static unsigned long
> +isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> +			unsigned long end_pfn, isolate_mode_t isolate_mode)
>  {
> -	unsigned long last_pageblock_nr = 0, pageblock_nr;
> +	struct zone *zone = cc->zone;
>  	unsigned long nr_scanned = 0, nr_isolated = 0;
>  	struct list_head *migratelist = &cc->migratepages;
>  	struct lruvec *lruvec;
>  	unsigned long flags;
>  	bool locked = false;
>  	struct page *page = NULL, *valid_page = NULL;
> -	bool set_unsuitable = true;
> -	const isolate_mode_t mode = (cc->mode == MIGRATE_ASYNC ?
> -					ISOLATE_ASYNC_MIGRATE : 0) |
> -				    (unevictable ? ISOLATE_UNEVICTABLE : 0);
>  
>  	/*
>  	 * Ensure that there are not too many pages isolated from the LRU
> @@ -515,19 +503,6 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  			}
>  		}
>  
> -		/*
> -		 * migrate_pfn does not necessarily start aligned to a
> -		 * pageblock. Ensure that pfn_valid is called when moving
> -		 * into a new MAX_ORDER_NR_PAGES range in case of large
> -		 * memory holes within the zone
> -		 */
> -		if ((low_pfn & (MAX_ORDER_NR_PAGES - 1)) == 0) {
> -			if (!pfn_valid(low_pfn)) {
> -				low_pfn += MAX_ORDER_NR_PAGES - 1;
> -				continue;
> -			}
> -		}
> -
>  		if (!pfn_valid_within(low_pfn))
>  			continue;
>  		nr_scanned++;
> @@ -545,28 +520,6 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  		if (!valid_page)
>  			valid_page = page;
>  
> -		/* If isolation recently failed, do not retry */
> -		pageblock_nr = low_pfn >> pageblock_order;
> -		if (last_pageblock_nr != pageblock_nr) {
> -			int mt;
> -
> -			last_pageblock_nr = pageblock_nr;
> -			if (!isolation_suitable(cc, page))
> -				goto next_pageblock;
> -
> -			/*
> -			 * For async migration, also only scan in MOVABLE
> -			 * blocks. Async migration is optimistic to see if
> -			 * the minimum amount of work satisfies the allocation
> -			 */
> -			mt = get_pageblock_migratetype(page);
> -			if (cc->mode == MIGRATE_ASYNC &&
> -			    !migrate_async_suitable(mt)) {
> -				set_unsuitable = false;
> -				goto next_pageblock;
> -			}
> -		}
> -
>  		/*
>  		 * Skip if free. page_order cannot be used without zone->lock
>  		 * as nothing prevents parallel allocations or buddy merging.
> @@ -601,8 +554,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  		 */
>  		if (PageTransHuge(page)) {
>  			if (!locked)
> -				goto next_pageblock;
> -			low_pfn += (1 << compound_order(page)) - 1;
> +				low_pfn = ALIGN(low_pfn + 1,
> +						pageblock_nr_pages) - 1;
> +			else
> +				low_pfn += (1 << compound_order(page)) - 1;
> +

Hmm, any reason not to always advance and align low_pfn to 
pageblock_nr_pages?  I don't see how pageblock_order > HPAGE_PMD_ORDER 
would make sense if encountering thp.

>  			continue;
>  		}
>  
> @@ -632,7 +588,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  		lruvec = mem_cgroup_page_lruvec(page, zone);
>  
>  		/* Try isolate the page */
> -		if (__isolate_lru_page(page, mode) != 0)
> +		if (__isolate_lru_page(page, isolate_mode) != 0)
>  			continue;
>  
>  		VM_BUG_ON_PAGE(PageTransCompound(page), page);
> @@ -651,11 +607,6 @@ isolate_success:
>  			++low_pfn;
>  			break;
>  		}
> -
> -		continue;
> -
> -next_pageblock:
> -		low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
>  	}
>  
>  	acct_isolated(zone, locked, cc);
> @@ -668,8 +619,7 @@ next_pageblock:
>  	 * if the whole pageblock was scanned without isolating any page.
>  	 */
>  	if (low_pfn == end_pfn)
> -		update_pageblock_skip(cc, valid_page, nr_isolated,
> -				      set_unsuitable, true);
> +		update_pageblock_skip(cc, valid_page, nr_isolated, true);
>  
>  	trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
>  
> @@ -680,15 +630,61 @@ next_pageblock:
>  	return low_pfn;
>  }
>  
> +/**
> + * isolate_migratepages_range() - isolate migrate-able pages in a PFN range
> + * @start_pfn: The first PFN to start isolating.
> + * @end_pfn:   The one-past-last PFN.

Need to specify @cc?

> + *
> + * Returns zero if isolation fails fatally due to e.g. pending signal.
> + * Otherwise, function returns one-past-the-last PFN of isolated page
> + * (which may be greater than end_pfn if end fell in a middle of a THP page).
> + */
> +unsigned long
> +isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
> +							unsigned long end_pfn)
> +{
> +	unsigned long pfn, block_end_pfn;
> +
> +	/* Scan block by block. First and last block may be incomplete */
> +	pfn = start_pfn;
> +	block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
> +
> +	for (; pfn < end_pfn; pfn = block_end_pfn,
> +				block_end_pfn += pageblock_nr_pages) {
> +
> +		block_end_pfn = min(block_end_pfn, end_pfn);
> +
> +		/* Skip whole pageblock in case of a memory hole */
> +		if (!pfn_valid(pfn))
> +			continue;
> +
> +		pfn = isolate_migratepages_block(cc, pfn, block_end_pfn,
> +							ISOLATE_UNEVICTABLE);
> +
> +		/*
> +		 * In case of fatal failure, release everything that might
> +		 * have been isolated in the previous iteration, and signal
> +		 * the failure back to caller.
> +		 */
> +		if (!pfn) {
> +			putback_movable_pages(&cc->migratepages);
> +			cc->nr_migratepages = 0;
> +			break;
> +		}
> +	}
> +
> +	return pfn;
> +}
> +
>  #endif /* CONFIG_COMPACTION || CONFIG_CMA */
>  #ifdef CONFIG_COMPACTION
>  /*
>   * Based on information in the current compact_control, find blocks
>   * suitable for isolating free pages from and then isolate them.
>   */
> -static void isolate_freepages(struct zone *zone,
> -				struct compact_control *cc)
> +static void isolate_freepages(struct compact_control *cc)
>  {
> +	struct zone *zone = cc->zone;
>  	struct page *page;
>  	unsigned long block_start_pfn;	/* start of current pageblock */
>  	unsigned long block_end_pfn;	/* end of current pageblock */
> @@ -806,7 +802,7 @@ static struct page *compaction_alloc(struct page *migratepage,
>  	 */
>  	if (list_empty(&cc->freepages)) {
>  		if (!cc->contended)
> -			isolate_freepages(cc->zone, cc);
> +			isolate_freepages(cc);
>  
>  		if (list_empty(&cc->freepages))
>  			return NULL;
> @@ -840,34 +836,81 @@ typedef enum {
>  } isolate_migrate_t;
>  
>  /*
> - * Isolate all pages that can be migrated from the block pointed to by
> - * the migrate scanner within compact_control.
> + * Isolate all pages that can be migrated from the first suitable block,
> + * starting at the block pointed to by the migrate scanner pfn within
> + * compact_control.
>   */
>  static isolate_migrate_t isolate_migratepages(struct zone *zone,
>  					struct compact_control *cc)
>  {
>  	unsigned long low_pfn, end_pfn;
> +	struct page *page;
> +	const isolate_mode_t isolate_mode =
> +		(cc->mode == MIGRATE_ASYNC ? ISOLATE_ASYNC_MIGRATE : 0);
>  
> -	/* Do not scan outside zone boundaries */
> -	low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);
> +	/*
> +	 * Start at where we last stopped, or beginning of the zone as
> +	 * initialized by compact_zone()
> +	 */
> +	low_pfn = cc->migrate_pfn;
>  
>  	/* Only scan within a pageblock boundary */
>  	end_pfn = ALIGN(low_pfn + 1, 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 ISOLATE_NONE;
> -	}
> +	/*
> +	 * Iterate over whole pageblocks until we find the first suitable.
> +	 * Do not cross the free scanner.
> +	 */
> +	for (; end_pfn <= cc->free_pfn;
> +			low_pfn = end_pfn, end_pfn += pageblock_nr_pages) {
> +
> +		/*
> +		 * This can potentially iterate a massively long zone with
> +		 * many pageblocks unsuitable, so periodically check if we
> +		 * need to schedule, or even abort async compaction.
> +		 */
> +		if (!(low_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
> +						&& compact_should_abort(cc))
> +			break;
> +
> +		/* Skip whole pageblock in case of a memory hole */
> +		if (!pfn_valid(low_pfn))
> +			continue;
> +
> +		page = pfn_to_page(low_pfn);
> +
> +		/* If isolation recently failed, do not retry */
> +		if (!isolation_suitable(cc, page))
> +			continue;
> +
> +		/*
> +		 * For async compaction, also only scan in MOVABLE blocks.
> +		 * Async compaction is optimistic to see if the minimum amount
> +		 * of work satisfies the allocation.
> +		 */
> +		if (cc->mode == MIGRATE_ASYNC &&
> +		    !migrate_async_suitable(get_pageblock_migratetype(page)))
> +			continue;
> +
> +		/* Perform the isolation */
> +		low_pfn = isolate_migratepages_block(cc, low_pfn, end_pfn,
> +								isolate_mode);

Hmm, why would we want to unconditionally set pageblock_skip if no pages 
could be isolated from a pageblock when 
isolate_mode == ISOLATE_ASYNC_MIGRATE?  It seems like it erroneously skip 
pageblocks for cases when isolate_mode == 0.

> +
> +		if (!low_pfn || cc->contended)
> +			return ISOLATE_ABORT;
>  
> -	/* Perform the isolation */
> -	low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn, false);
> -	if (!low_pfn || cc->contended)
> -		return ISOLATE_ABORT;
> +		/*
> +		 * Either we isolated something and proceed with migration. Or
> +		 * we failed and compact_zone should decide if we should
> +		 * continue or not.
> +		 */
> +		break;
> +	}
>  
> +	/* Record where migration scanner will be restarted */
>  	cc->migrate_pfn = low_pfn;
>  
> -	return ISOLATE_SUCCESS;
> +	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
>  }
>  
>  static int compact_finished(struct zone *zone,
> @@ -1040,9 +1083,6 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>  			;
>  		}
>  
> -		if (!cc->nr_migratepages)
> -			continue;
> -
>  		err = migrate_pages(&cc->migratepages, compaction_alloc,
>  				compaction_free, (unsigned long)cc, cc->mode,
>  				MR_COMPACTION);
> diff --git a/mm/internal.h b/mm/internal.h
> index a1b651b..5a0738f 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -154,8 +154,8 @@ unsigned long
>  isolate_freepages_range(struct compact_control *cc,
>  			unsigned long start_pfn, unsigned long end_pfn);
>  unsigned long
> -isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> -	unsigned long low_pfn, unsigned long end_pfn, bool unevictable);
> +isolate_migratepages_range(struct compact_control *cc,
> +			   unsigned long low_pfn, unsigned long end_pfn);
>  
>  #endif
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 91191fb..f424752 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6292,8 +6292,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>  
>  		if (list_empty(&cc->migratepages)) {
>  			cc->nr_migratepages = 0;
> -			pfn = isolate_migratepages_range(cc->zone, cc,
> -							 pfn, end, true);
> +			pfn = isolate_migratepages_range(cc, pfn, end);
>  			if (!pfn) {
>  				ret = -EINTR;
>  				break;
--
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