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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/2/24 17:15, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> 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@nvidia.com>

...

>  /*
> @@ -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).

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.

> +};
>  /*
>   * 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.