Re: [RFC PATCH 1/4] mm/compaction: add support for >0 order folio memory compaction.

From: Zi Yan
Date: Mon Sep 18 2023 - 13:06:40 EST

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

> On 9/13/2023 12:28 AM, 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 | 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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