[RFC] vmap_folio()

[Matthew Wilcox <willy@infradead.org>]

On Tue, Aug 16, 2022 at 07:08:22PM +0100, Matthew Wilcox wrote:
> For these reasons, I proposing the logical equivalent to this:
> 
> +void *folio_map_local(struct folio *folio)
> +{
> +       if (!IS_ENABLED(CONFIG_HIGHMEM))
> +               return folio_address(folio);
> +       if (!folio_test_large(folio))
> +               return kmap_local_page(&folio->page);
> +       return vmap_folio(folio);
> +}
> 
> (where vmap_folio() is a new function that works a lot like vmap(),
> chunks of this get moved out-of-line, etc, etc., but this concept)

This vmap_folio() compiles but is otherwise untested.  Anything I
obviously got wrong here?

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index dd6cdb201195..1867759c33ff 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2848,6 +2848,42 @@ void *vmap(struct page **pages, unsigned int count,
 }
 EXPORT_SYMBOL(vmap);
 
+#ifdef CONFIG_HIGHMEM
+/**
+ * vmap_folio - Map an entire folio into virtually contiguous space
+ * @folio: The folio to map.
+ *
+ * Maps all pages in @folio into contiguous kernel virtual space.  This
+ * function is only available in HIGHMEM builds; for !HIGHMEM, use
+ * folio_address().  The pages are mapped with PAGE_KERNEL permissions.
+ *
+ * Return: The address of the area or %NULL on failure
+ */
+void *vmap_folio(struct folio *folio)
+{
+	size_t size = folio_size(folio);
+	struct vm_struct *area;
+	unsigned long addr;
+
+	might_sleep();
+
+	area = get_vm_area_caller(size, VM_MAP, __builtin_return_address(0));
+	if (!area)
+		return NULL;
+
+	addr = (unsigned long)area->addr;
+	if (vmap_range_noflush(addr, addr + size,
+				folio_pfn(folio) << PAGE_SHIFT,
+				PAGE_KERNEL, folio_shift(folio))) {
+		vunmap(area->addr);
+		return NULL;
+	}
+	flush_cache_vmap(addr, addr + size);
+
+	return area->addr;
+}
+#endif
+
 #ifdef CONFIG_VMAP_PFN
 struct vmap_pfn_data {
 	unsigned long	*pfns;