Re: [RFC PATCH 2/4] mm/compaction: optimize >0 order folio compaction with free page split.

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/13/2023 12:28 AM, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> During migration in a memory compaction, free pages are placed in an array
> of page lists based on their order. But the desired free page order (i.e.,
> the order of a source page) might not be always present, thus leading to
> migration failures. Split a high order free pages when source migration
> page has a lower order to increase migration successful rate.
> 
> Note: merging free pages when a migration fails and a lower order free
> page is returned via compaction_free() is possible, but there is too much
> work. Since the free pages are not buddy pages, it is hard to identify
> these free pages using existing PFN-based page merging algorithm.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>   mm/compaction.c | 40 +++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 868e92e55d27..45747ab5f380 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1801,9 +1801,46 @@ 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);
> +	bool has_isolated_pages = false;
>   
> +again:
>   	if (!cc->freepages[order].nr_free) {
> -		isolate_freepages(cc);
> +		int i;
> +
> +		for (i = order + 1; i <= MAX_ORDER; i++) {
> +			if (cc->freepages[i].nr_free) {
> +				struct page *freepage =
> +					list_first_entry(&cc->freepages[i].pages,
> +							 struct page, lru);
> +
> +				int start_order = i;
> +				unsigned long size = 1 << start_order;
> +
> +				list_del(&freepage->lru);
> +				cc->freepages[i].nr_free--;
> +
> +				while (start_order > order) {
> +					start_order--;
> +					size >>= 1;
> +
> +					list_add(&freepage[size].lru,
> +						&cc->freepages[start_order].pages);
> +					cc->freepages[start_order].nr_free++;
> +					set_page_private(&freepage[size], start_order);

IIUC, these split pages should also call functions to initialize? e.g. 
prep_compound_page()?

> +				}
> +				post_alloc_hook(freepage, order, __GFP_MOVABLE);
> +				if (order)
> +					prep_compound_page(freepage, order);
> +				dst = page_folio(freepage);
> +				goto done;
> +			}
> +		}
> +		if (!has_isolated_pages) {
> +			isolate_freepages(cc);
> +			has_isolated_pages = true;
> +			goto again;
> +		}
> +
>   		if (!cc->freepages[order].nr_free)
>   			return NULL;
>   	}
> @@ -1814,6 +1851,7 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>   	post_alloc_hook(&dst->page, order, __GFP_MOVABLE);
>   	if (order)
>   		prep_compound_page(&dst->page, order);
> +done:
>   	cc->nr_freepages -= 1 << order;
>   	return dst;
>   }