Date:   Mon, 18 Sep 2023 13:06:25 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "\"Matthew Wilcox (Oracle)\"" <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        "\"Yin, Fengwei\"" <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Kemeng Shi <shikemeng@...weicloud.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rohan Puri <rohan.puri15@...il.com>,
        Mcgrof Chamberlain <mcgrof@...nel.org>,
        Adam Manzanares <a.manzanares@...sung.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC PATCH 1/4] mm/compaction: add support for >0 order folio
 memory compaction.

On 15 Sep 2023, at 5:33, Baolin Wang wrote:

> On 9/13/2023 12:28 AM, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> 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@...dia.com>
>> ---
>>   mm/compaction.c | 108 +++++++++++++++++++++++++++++++++++++++---------
>>   mm/internal.h   |   7 +++-
>>   2 files changed, 94 insertions(+), 21 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 01ba298739dd..868e92e55d27 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -107,6 +107,44 @@ static void split_map_pages(struct list_head *list)
>>   	list_splice(&tmp_list, list);
>>   }
>>  +static unsigned long release_free_list(struct free_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;
>> +		}
>> +	}
>> +	return high_pfn;
>> +}
>> +
>> +static void sort_free_pages(struct list_head *src, struct free_list *dst)
>> +{
>> +	unsigned int order;
>> +	struct page *page, *next;
>> +
>> +	list_for_each_entry_safe(page, next, src, lru) {
>> +		order = buddy_order(page);
>
> These pages are already isolated from the buddy system, but continue to use buddy_order() to get the page order, which can be confused. Moreover, the buddy_order() should be under the zone lock protection.
>
> IMO, just use 'page_private()' to get the page order like split_map_pages() already did, that seems more readable.

Sure. Will do in the next version. Thanks.

>
>> +
>> +		list_move(&page->lru, &dst[order].pages);
>> +		dst[order].nr_free++;
>> +	}
>> +}
>> +
>>   #ifdef CONFIG_COMPACTION
>>   bool PageMovable(struct page *page)
>>   {
>> @@ -1422,6 +1460,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn)
>>   {
>>   	unsigned long start_pfn, end_pfn;
>>   	struct page *page;
>> +	LIST_HEAD(freelist);
>>    	/* Do not search around if there are enough pages already */
>>   	if (cc->nr_freepages >= cc->nr_migratepages)
>> @@ -1439,7 +1478,8 @@ 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, &freelist, 1, false);
>> +	sort_free_pages(&freelist, cc->freepages);
>>    	/* Skip this pageblock in the future as it's full or nearly full */
>>   	if (start_pfn == end_pfn && !cc->no_set_skip_hint)
>> @@ -1568,7 +1608,7 @@ 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);
>
> Missed to update cc->freepages[order].nr_free?

Good catch. Thanks. Will fix it.

>
>>   				count_compact_events(COMPACTISOLATED, nr_isolated);
>>   			} else {
>>   				/* If isolation fails, abort the search */
>> @@ -1642,13 +1682,13 @@ 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;
>> +	LIST_HEAD(freelist);
>>    	/* 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
>> @@ -1708,7 +1748,8 @@ 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, &freelist, stride, false);
>> +		sort_free_pages(&freelist, cc->freepages);
>>    		/* Update the skip hint if the full pageblock was scanned */
>>   		if (isolate_start_pfn == block_end_pfn)
>> @@ -1749,10 +1790,6 @@ 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);
>>   }
>>    /*
>> @@ -1763,18 +1800,21 @@ 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);
>>  -	if (list_empty(&cc->freepages)) {
>> +	if (!cc->freepages[order].nr_free) {
>>   		isolate_freepages(cc);
>> -
>> -		if (list_empty(&cc->freepages))
>> +		if (!cc->freepages[order].nr_free)
>>   			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_free--;
>>   	list_del(&dst->lru);
>> -	cc->nr_freepages--;
>> -
>> +	post_alloc_hook(&dst->page, order, __GFP_MOVABLE);
>> +	if (order)
>> +		prep_compound_page(&dst->page, order);
>> +	cc->nr_freepages -= 1 << order;
>>   	return dst;
>>   }
>>  @@ -1786,9 +1826,34 @@ 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;
>>  -	list_add(&dst->lru, &cc->freepages);
>> -	cc->nr_freepages++;
>> +	if (order) {
>> +		int i;
>> +
>> +		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_free++;
>> +	cc->nr_freepages += 1 << order;
>>   }
>>    /* possible outcome of isolate_migratepages */
>> @@ -2412,6 +2477,7 @@ 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
>> @@ -2421,7 +2487,10 @@ 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_LIST_HEAD(&cc->freepages[order].pages);
>> +		cc->freepages[order].nr_free = 0;
>> +	}
>>   	INIT_LIST_HEAD(&cc->migratepages);
>>    	cc->migratetype = gfp_migratetype(cc->gfp_mask);
>> @@ -2607,7 +2676,7 @@ 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);
>> @@ -2626,7 +2695,6 @@ 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 8c90e966e9f8..f5c691bb5c1c 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -465,6 +465,11 @@ int split_free_page(struct page *free_page,
>>   /*
>>    * in mm/compaction.c
>>    */
>> +
>> +struct free_list {
>> +	struct list_head	pages;
>> +	unsigned long		nr_free;
>> +};
>>   /*
>>    * 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
>> @@ -473,7 +478,7 @@ 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 free_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 */


--
Best Regards,
Yan, Zi

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