Re: [PATCH v3 2/3] mm/compaction: add support for >0 order folio memory compaction.

[Zi Yan]

On 9 Feb 2024, at 11:37, Vlastimil Babka wrote:

> On 2/2/24 17:15, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> Before last commit, memory compaction only migrates order-0 folios and
>> skips >0 order folios. Last commit splits all >0 order folios during
>> compaction. This commit migrates >0 order folios during 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 NR_PAGE_ORDERS 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 using newly added
>>    free_pages_prepare_fpi_none().
>>
>> Step 3 is done for a latter optimization that splitting and/or merging free
>> pages during compaction becomes easier.
>>
>> Note: without splitting free pages, compaction can end prematurely due to
>> migration will return -ENOMEM even if there is free pages. This happens
>> when no order-0 free page exist and compaction_alloc() return NULL.
>>
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>
> ...
>
>>  /*
>> @@ -1835,9 +1857,17 @@ 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;
>> +
>> +	folio_set_count(dst, 0);
>
> We can't change refcount to 0 like this, after it was already set to 1 and
> somebody else might have done get_page_unless_zero(). You need to either
> put_page_testzero() and if it's false, consider the page lost, or leave it
> refcounted and adjust the code to handle both refcounted and non-refcounted
> pages on the lists (the first option is simpler and shouldn't be too bad).
Got it. Will fix it with the first option. Thanks.

>
> Perhaps folio_set_count()/set_page_count() should get some comment warning
> against this kind of mistake.
>
>> +	free_pages_prepare_fpi_none(page, order);
>>
>> -	list_add(&dst->lru, &cc->freepages);
>> -	cc->nr_freepages++;
>> +	INIT_LIST_HEAD(&dst->lru);
>> +
>> +	list_add(&dst->lru, &cc->freepages[order].pages);
>> +	cc->freepages[order].nr_pages++;
>> +	cc->nr_freepages += 1 << order;
>>  }
>>
>
> ...
>
>>
>>  extern void free_unref_page(struct page *page, unsigned int order);
>> @@ -473,6 +475,11 @@ int split_free_page(struct page *free_page,
>>  /*
>>   * in mm/compaction.c
>>   */
>> +
>> +struct page_list {
>> +	struct list_head	pages;
>> +	unsigned long		nr_pages;
>
> I've checked and even with patch 3/3 I don't think you actually need the
> counter? The only check of the counter I noticed was to check for
> zero/non-zero, and you could use list_empty() instead.

Sure. I will remove 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 +488,7 @@ 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[NR_PAGE_ORDERS];	/* 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 */
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 5be4cd8f6b5a..c7c135e6d5ee 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1179,6 +1179,12 @@ static __always_inline bool free_pages_prepare(struct page *page,
>>  	return true;
>>  }
>>
>> +__always_inline bool free_pages_prepare_fpi_none(struct page *page,
>> +			unsigned int order)
>> +{
>> +	return free_pages_prepare(page, order, FPI_NONE);
>> +}
>> +
>>  /*
>>   * Frees a number of pages from the PCP lists
>>   * Assumes all pages on list are in same zone.


--
Best Regards,
Yan, Zi

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