Re: [PATCH v5 2/3] mm/compaction: add support for >0 order folio memory compaction.

[Vlastimil Babka]

On 2/14/24 23:04, 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>
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Tested-by: Yu Zhao <yuzhao@xxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Noticed a possible simplification:

> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -447,6 +447,8 @@ extern void prep_compound_page(struct page *page, unsigned int order);
>  
>  extern void post_alloc_hook(struct page *page, unsigned int order,
>  					gfp_t gfp_flags);
> +extern bool free_pages_prepare_fpi_none(struct page *page, unsigned int order);
> +
>  extern int user_min_free_kbytes;
>  
>  extern void free_unref_page(struct page *page, unsigned int order);
> @@ -481,7 +483,7 @@ 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 list_head 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 7ae4b74c9e5c..e6e2ac722a82 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);

Seems like free_pages_prepare() currently only passes  fpi_flags to
should_skip_kasan_poison() and that ignores them. You could remove the
parameter from both and declare and use free_pages_prepare(page, order)
directly.

> +}
> +
>  /*
>   * Frees a number of pages from the PCP lists
>   * Assumes all pages on list are in same zone.