Today with folios being an overlay on struct page, folio_alloc() is simple; we allocate a compound page and cast the result to a struct folio pointer.
Once we live in a MatthewWilcox/Memdescs world, we need to do two allocations. First, we allocate a struct folio from slab. Then we call into the MatthewWilcox/BuddyAllocator to allocate the memory, which will set the memdesc value in each page to refer to the folio. I envisage a function like this:
struct folio *folio_alloc(gfp_t gfp, unsigned int order)
{
struct page *page;
struct folio *folio = kmem_cache_alloc(folio_slab, gfp);
if (!folio)
return NULL;
page = alloc_page_desc(gfp, order, MEMDESC_TYPE_FOLIO, folio);
if (!page) {
kmem_cache_free(folio_slab, folio);
return NULL;
}
folio->pfn = page_to_pfn(page);
folio->order = order;
folio->flags = pfn_flags(folio->pfn);
init_list_head(&folio->lru);
folio->mapping = NULL;
folio->private = NULL;
atomic_set(&folio->_mapcount, -1);
folio->memcg_data = 0;
if (order > 0) {
atomic_set(&folio->_entire_mapcount, 0);
atomic_set(&folio->_nr_pages_mapped, 0);
init_list_head(&folio->_deferred_split_list);
}
atomic_set(&folio->_refcount, 1);
return folio;
}