Re: [PATCH v1 2/4] mm/compaction: add support for >0 order folio memory compaction.

[Ryan Roberts]

On 13/11/2023 17:01, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
> 
> Before, memory compaction only migrates order-0 folios and skips >0 order
> folios. This commit adds support for >0 order folio compaction by keeping
> isolated free pages at their original size without splitting them into
> order-0 pages and using them directly during migration process.
> 
> What is different from the prior implementation:
> 1. All isolated free pages are kept in a MAX_ORDER+1 array of page lists,
>    where each page list stores free pages in the same order.
> 2. All free pages are not post_alloc_hook() processed nor buddy pages,
>    although their orders are stored in first page's private like buddy
>    pages.
> 3. During migration, in new page allocation time (i.e., in
>    compaction_alloc()), free pages are then processed by post_alloc_hook().
>    When migration fails and a new page is returned (i.e., in
>    compaction_free()), free pages are restored by reversing the
>    post_alloc_hook() operations.
> 
> Step 3 is done for a latter optimization that splitting and/or merging free
> pages during compaction becomes easier.
> 
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>  mm/compaction.c | 160 ++++++++++++++++++++++++++++++------------------
>  mm/internal.h   |   7 ++-
>  2 files changed, 108 insertions(+), 59 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 5217dd35b493..ec6b5cc7e907 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -66,45 +66,64 @@ static inline void count_compact_events(enum vm_event_item item, long delta)
>  #define COMPACTION_HPAGE_ORDER	(PMD_SHIFT - PAGE_SHIFT)
>  #endif
>  
> -static unsigned long release_freepages(struct list_head *freelist)
> +static void init_page_list(struct page_list *p)
>  {
> -	struct page *page, *next;
> -	unsigned long high_pfn = 0;
> -
> -	list_for_each_entry_safe(page, next, freelist, lru) {
> -		unsigned long pfn = page_to_pfn(page);
> -		list_del(&page->lru);
> -		__free_page(page);
> -		if (pfn > high_pfn)
> -			high_pfn = pfn;
> -	}
> -
> -	return high_pfn;
> +	INIT_LIST_HEAD(&p->pages);
> +	p->nr_pages = 0;
>  }
>  
> -static void split_map_pages(struct list_head *list)
> +static void split_map_pages(struct page_list *freepages)
>  {
>  	unsigned int i, order, nr_pages;
>  	struct page *page, *next;
>  	LIST_HEAD(tmp_list);
>  
> -	list_for_each_entry_safe(page, next, list, lru) {
> -		list_del(&page->lru);
> +	for (order = 0; order <= MAX_ORDER; order++) {
> +		freepages[order].nr_pages = 0;
> +
> +		list_for_each_entry_safe(page, next, &freepages[order].pages, lru) {
> +			list_del(&page->lru);
>  
> -		order = page_private(page);
> -		nr_pages = 1 << order;
> +			nr_pages = 1 << order;
>  
> -		post_alloc_hook(page, order, __GFP_MOVABLE);
> -		if (order)
> -			split_page(page, order);
> +			post_alloc_hook(page, order, __GFP_MOVABLE);
> +			if (order)
> +				split_page(page, order);
>  
> -		for (i = 0; i < nr_pages; i++) {
> -			list_add(&page->lru, &tmp_list);
> -			page++;
> +			for (i = 0; i < nr_pages; i++) {
> +				list_add(&page->lru, &tmp_list);
> +				page++;
> +			}
> +			freepages[order].nr_pages += nr_pages;
>  		}
> +		list_splice(&tmp_list, &freepages[order].pages);

I think this should be list_splice_init() since you are reusing tmp_list in each
iteration of the outer loop?

Thanks,
Ryan

>  	}
> +}
>  
> -	list_splice(&tmp_list, list);
> +static unsigned long release_free_list(struct page_list *freepages)
> +{
> +	int order;
> +	unsigned long high_pfn = 0;
> +
> +	for (order = 0; order <= MAX_ORDER; order++) {
> +		struct page *page, *next;
> +
> +		list_for_each_entry_safe(page, next, &freepages[order].pages, lru) {
> +			unsigned long pfn = page_to_pfn(page);
> +
> +			list_del(&page->lru);
> +			/*
> +			 * Convert free pages into post allocation pages, so
> +			 * that we can free them via __free_page.
> +			 */
> +			post_alloc_hook(page, order, __GFP_MOVABLE);
> +			__free_pages(page, order);
> +			if (pfn > high_pfn)
> +				high_pfn = pfn;
> +		}
> +		freepages[order].nr_pages = 0;
> +	}
> +	return high_pfn;
>  }
>  
>  #ifdef CONFIG_COMPACTION
> @@ -583,7 +602,7 @@ static bool compact_unlock_should_abort(spinlock_t *lock,
>  static unsigned long isolate_freepages_block(struct compact_control *cc,
>  				unsigned long *start_pfn,
>  				unsigned long end_pfn,
> -				struct list_head *freelist,
> +				struct page_list *freelist,
>  				unsigned int stride,
>  				bool strict)
>  {
> @@ -657,7 +676,8 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  		nr_scanned += isolated - 1;
>  		total_isolated += isolated;
>  		cc->nr_freepages += isolated;
> -		list_add_tail(&page->lru, freelist);
> +		list_add_tail(&page->lru, &freelist[order].pages);
> +		freelist[order].nr_pages++;
>  
>  		if (!strict && cc->nr_migratepages <= cc->nr_freepages) {
>  			blockpfn += isolated;
> @@ -722,7 +742,11 @@ isolate_freepages_range(struct compact_control *cc,
>  			unsigned long start_pfn, unsigned long end_pfn)
>  {
>  	unsigned long isolated, pfn, block_start_pfn, block_end_pfn;
> -	LIST_HEAD(freelist);
> +	int order;
> +	struct page_list tmp_freepages[MAX_ORDER + 1];
> +
> +	for (order = 0; order <= MAX_ORDER; order++)
> +		init_page_list(&tmp_freepages[order]);
>  
>  	pfn = start_pfn;
>  	block_start_pfn = pageblock_start_pfn(pfn);
> @@ -753,7 +777,7 @@ isolate_freepages_range(struct compact_control *cc,
>  			break;
>  
>  		isolated = isolate_freepages_block(cc, &isolate_start_pfn,
> -					block_end_pfn, &freelist, 0, true);
> +					block_end_pfn, tmp_freepages, 0, true);
>  
>  		/*
>  		 * In strict mode, isolate_freepages_block() returns 0 if
> @@ -770,15 +794,15 @@ isolate_freepages_range(struct compact_control *cc,
>  		 */
>  	}
>  
> -	/* __isolate_free_page() does not map the pages */
> -	split_map_pages(&freelist);
> -
>  	if (pfn < end_pfn) {
>  		/* Loop terminated early, cleanup. */
> -		release_freepages(&freelist);
> +		release_free_list(tmp_freepages);
>  		return 0;
>  	}
>  
> +	/* __isolate_free_page() does not map the pages */
> +	split_map_pages(tmp_freepages);
> +
>  	/* We don't use freelists for anything. */
>  	return pfn;
>  }
> @@ -1462,7 +1486,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn)
>  	if (!page)
>  		return;
>  
> -	isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);
> +	isolate_freepages_block(cc, &start_pfn, end_pfn, cc->freepages, 1, false);
>  
>  	/* Skip this pageblock in the future as it's full or nearly full */
>  	if (start_pfn == end_pfn && !cc->no_set_skip_hint)
> @@ -1591,7 +1615,7 @@ static void fast_isolate_freepages(struct compact_control *cc)
>  				nr_scanned += nr_isolated - 1;
>  				total_isolated += nr_isolated;
>  				cc->nr_freepages += nr_isolated;
> -				list_add_tail(&page->lru, &cc->freepages);
> +				list_add_tail(&page->lru, &cc->freepages[order].pages);
>  				count_compact_events(COMPACTISOLATED, nr_isolated);
>  			} else {
>  				/* If isolation fails, abort the search */
> @@ -1665,13 +1689,12 @@ static void isolate_freepages(struct compact_control *cc)
>  	unsigned long isolate_start_pfn; /* exact pfn we start at */
>  	unsigned long block_end_pfn;	/* end of current pageblock */
>  	unsigned long low_pfn;	     /* lowest pfn scanner is able to scan */
> -	struct list_head *freelist = &cc->freepages;
>  	unsigned int stride;
>  
>  	/* Try a small search of the free lists for a candidate */
>  	fast_isolate_freepages(cc);
>  	if (cc->nr_freepages)
> -		goto splitmap;
> +		return;
>  
>  	/*
>  	 * Initialise the free scanner. The starting point is where we last
> @@ -1731,7 +1754,7 @@ static void isolate_freepages(struct compact_control *cc)
>  
>  		/* Found a block suitable for isolating free pages from. */
>  		nr_isolated = isolate_freepages_block(cc, &isolate_start_pfn,
> -					block_end_pfn, freelist, stride, false);
> +					block_end_pfn, cc->freepages, stride, false);
>  
>  		/* Update the skip hint if the full pageblock was scanned */
>  		if (isolate_start_pfn == block_end_pfn)
> @@ -1772,10 +1795,6 @@ static void isolate_freepages(struct compact_control *cc)
>  	 * and the loop terminated due to isolate_start_pfn < low_pfn
>  	 */
>  	cc->free_pfn = isolate_start_pfn;
> -
> -splitmap:
> -	/* __isolate_free_page() does not map the pages */
> -	split_map_pages(freelist);
>  }
>  
>  /*
> @@ -1786,23 +1805,22 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>  {
>  	struct compact_control *cc = (struct compact_control *)data;
>  	struct folio *dst;
> +	int order = folio_order(src);
>  
> -	/* this makes migrate_pages() split the source page and retry */
> -	if (folio_order(src) > 0)
> -		return NULL;
> -
> -	if (list_empty(&cc->freepages)) {
> +	if (!cc->freepages[order].nr_pages) {
>  		isolate_freepages(cc);
> -
> -		if (list_empty(&cc->freepages))
> +		if (!cc->freepages[order].nr_pages)
>  			return NULL;
>  	}
>  
> -	dst = list_entry(cc->freepages.next, struct folio, lru);
> +	dst = list_first_entry(&cc->freepages[order].pages, struct folio, lru);
> +	cc->freepages[order].nr_pages--;
>  	list_del(&dst->lru);
> -	cc->nr_freepages--;
> -
> -	return dst;
> +	post_alloc_hook(&dst->page, order, __GFP_MOVABLE);
> +	if (order)
> +		prep_compound_page(&dst->page, order);
> +	cc->nr_freepages -= 1 << order;
> +	return page_rmappable_folio(&dst->page);
>  }
>  
>  /*
> @@ -1813,9 +1831,34 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>  static void compaction_free(struct folio *dst, unsigned long data)
>  {
>  	struct compact_control *cc = (struct compact_control *)data;
> +	int order = folio_order(dst);
> +	struct page *page = &dst->page;
> +
> +	if (order) {
> +		int i;
>  
> -	list_add(&dst->lru, &cc->freepages);
> -	cc->nr_freepages++;
> +		page[1].flags &= ~PAGE_FLAGS_SECOND;
> +		for (i = 1; i < (1 << order); i++) {
> +			page[i].mapping = NULL;
> +			clear_compound_head(&page[i]);
> +			page[i].flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> +		}
> +
> +	}
> +	/* revert post_alloc_hook() operations */
> +	page->mapping = NULL;
> +	page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> +	set_page_count(page, 0);
> +	page_mapcount_reset(page);
> +	reset_page_owner(page, order);
> +	page_table_check_free(page, order);
> +	arch_free_page(page, order);
> +	set_page_private(page, order);
> +	INIT_LIST_HEAD(&dst->lru);
> +
> +	list_add(&dst->lru, &cc->freepages[order].pages);
> +	cc->freepages[order].nr_pages++;
> +	cc->nr_freepages += 1 << order;
>  }
>  
>  /* possible outcome of isolate_migratepages */
> @@ -2439,6 +2482,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>  	const bool sync = cc->mode != MIGRATE_ASYNC;
>  	bool update_cached;
>  	unsigned int nr_succeeded = 0;
> +	int order;
>  
>  	/*
>  	 * These counters track activities during zone compaction.  Initialize
> @@ -2448,7 +2492,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>  	cc->total_free_scanned = 0;
>  	cc->nr_migratepages = 0;
>  	cc->nr_freepages = 0;
> -	INIT_LIST_HEAD(&cc->freepages);
> +	for (order = 0; order <= MAX_ORDER; order++)
> +		init_page_list(&cc->freepages[order]);
>  	INIT_LIST_HEAD(&cc->migratepages);
>  
>  	cc->migratetype = gfp_migratetype(cc->gfp_mask);
> @@ -2634,7 +2679,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>  	 * so we don't leave any returned pages behind in the next attempt.
>  	 */
>  	if (cc->nr_freepages > 0) {
> -		unsigned long free_pfn = release_freepages(&cc->freepages);
> +		unsigned long free_pfn = release_free_list(cc->freepages);
>  
>  		cc->nr_freepages = 0;
>  		VM_BUG_ON(free_pfn == 0);
> @@ -2653,7 +2698,6 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>  
>  	trace_mm_compaction_end(cc, start_pfn, end_pfn, sync, ret);
>  
> -	VM_BUG_ON(!list_empty(&cc->freepages));
>  	VM_BUG_ON(!list_empty(&cc->migratepages));
>  
>  	return ret;
> diff --git a/mm/internal.h b/mm/internal.h
> index 8450562744cf..46c8bb94ecbb 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -473,6 +473,11 @@ int split_free_page(struct page *free_page,
>  /*
>   * in mm/compaction.c
>   */
> +
> +struct page_list {
> +	struct list_head	pages;
> +	unsigned long		nr_pages;
> +};
>  /*
>   * compact_control is used to track pages being migrated and the free pages
>   * they are being migrated to during memory compaction. The free_pfn starts
> @@ -481,7 +486,7 @@ int split_free_page(struct page *free_page,
>   * completes when free_pfn <= migrate_pfn
>   */
>  struct compact_control {
> -	struct list_head freepages;	/* List of free pages to migrate to */
> +	struct page_list freepages[MAX_ORDER + 1];	/* List of free pages to migrate to */
>  	struct list_head migratepages;	/* List of pages being migrated */
>  	unsigned int nr_freepages;	/* Number of isolated free pages */
>  	unsigned int nr_migratepages;	/* Number of pages to migrate */