Re: [PATCH v3 1/6] mm/thp: add prep_transhuge_device_private_page()

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 05, 2020 at 04:51:42PM -0800, Ralph Campbell wrote:
> Add a helper function to allow device drivers to create device private
> transparent huge pages. This is intended to help support device private
> THP migrations.

I think you'd be better off with these calling conventions:

-void prep_transhuge_page(struct page *page)
+struct page *thp_prep(struct page *page)
 {
+       if (!page || compound_order(page) == 0)
+               return page;
        /*
-        * we use page->mapping and page->indexlru in second tail page
+        * we use page->mapping and page->index in second tail page
         * as list_head: assuming THP order >= 2
         */
+       BUG_ON(compound_order(page) == 1);
 
        INIT_LIST_HEAD(page_deferred_list(page));
        set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
+
+       return page;
 }

It simplifies the users.

> +void prep_transhuge_device_private_page(struct page *page)
> +{
> +	prep_compound_page(page, HPAGE_PMD_ORDER);
> +	prep_transhuge_page(page);
> +	/* Only the head page has a reference to the pgmap. */
> +	percpu_ref_put_many(page->pgmap->ref, HPAGE_PMD_NR - 1);
> +}
> +EXPORT_SYMBOL_GPL(prep_transhuge_device_private_page);

Something else that may interest you from my patch series is support
for page sizes other than PMD_SIZE.  I don't know what page sizes your
hardware supports.  There's no support for page sizes other than PMD
for anonymous memory, so this might not be too useful for you yet.