On 11/6/20 4:14 AM, Matthew Wilcox wrote:
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.
I'm not sure what the simplification is. If you mean the name change from prep_transhuge_page() to thp_prep(), that seems fine to me. The following could also be renamed to thp_prep_device_private_page() or similar.
+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 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.
I did see those changes. It might help some device drivers to do DMA in larger than PAGE_SIZE blocks but less than PMD_SIZE. It might help reduce page table sizes since 2MB, 64K, and 4K are commonly supported GPU page sizes. The MIGRATE_PFN_COMPOUND flag is intended to indicate that the page size is determined by page_size() so I was thinking ahead to other than PMD sized pages. However, when migrating a pte_none() or pmd_none() page, there is no source page to determine the size. Maybe I need to encode the page order in the migrate PFN entry like hmm_range_fault().
Anyway, I agree that thinking about page sizes other than PMD is good.