Re: [PATCH v1 3/4] mm/compaction: optimize >0 order folio compaction with free page split.

Ryan Roberts
2023-11-22T10:26:37Z

On 13/11/2023 17:01, 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 ec6b5cc7e907..9c083e6b399a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1806,9 +1806,46 @@ 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_pages) {
> - isolate_freepages(cc);
> + int i;
> +
> + for (i = order + 1; i <= MAX_ORDER; i++) {
> + if (cc->freepages[i].nr_pages) {
> + 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_pages--;
> +
> + while (start_order > order) {
> + start_order--;
> + size >>= 1;
> +
> + list_add(&freepage[size].lru,
> + &cc->freepages[start_order].pages);
> + cc->freepages[start_order].nr_pages++;
> + set_page_private(&freepage[size], start_order);
> + }
> + post_alloc_hook(freepage, order, __GFP_MOVABLE);
> + if (order)
> + prep_compound_page(freepage, order);
> + dst = page_folio(freepage);
> + goto done;

Perhaps just do:

dst = (struct folio *)freepage;
goto done;

then move done: up a couple of statements below, so that post_alloc_hook() and
prep_compound_page() are always done below in common path? Although perhaps the
cast is frowned upon, you're already making the assumption that page and folio
are interchangable the way you call list_first_entry().

> + }
> + }
> + if (!has_isolated_pages) {
> + isolate_freepages(cc);
> + has_isolated_pages = true;
> + goto again;
> + }
> +
> if (!cc->freepages[order].nr_pages)
> return NULL;
> }
> @@ -1819,6 +1856,7 @@ 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 page_rmappable_folio(&dst->page);
> }