Re: [PATCH 3/6] drm/i915: use vmap in shmem_pin_map

[Matthew Wilcox <willy@infradead.org>]

On Fri, Sep 18, 2020 at 06:37:21PM +0200, Christoph Hellwig wrote:
>  void shmem_unpin_map(struct file *file, void *ptr)
>  {
> +	long i = shmem_npages(file);
> +
>  	mapping_clear_unevictable(file->f_mapping);
> -	__shmem_unpin_map(file, ptr, shmem_npte(file));
> +	vunmap(ptr);
> +
> +	for (i = 0; i < shmem_npages(file); i++) {
> +		struct page *page;
> +
> +		page = shmem_read_mapping_page_gfp(file->f_mapping, i,
> +						   GFP_KERNEL);
> +		if (!WARN_ON(IS_ERR(page))) {
> +			put_page(page);
> +			put_page(page);
> +		}
> +	}
>  }

This is awkward.  I'd like it if we had a vfree() variant which called
put_page() instead of __free_pages().  I'd like it even more if we
used release_pages() instead of our own loop that called put_page().

Perhaps something like this ...

+++ b/mm/vmalloc.c
@@ -2262,7 +2262,7 @@ static void __vunmap(const void *addr, int deallocate_page
s)
 
        vm_remove_mappings(area, deallocate_pages);
 
-       if (deallocate_pages) {
+       if (deallocate_pages == 1) {
                int i;
 
                for (i = 0; i < area->nr_pages; i++) {
@@ -2271,8 +2271,12 @@ static void __vunmap(const void *addr, int deallocate_pages)
                        BUG_ON(!page);
                        __free_pages(page, 0);
                }
-               atomic_long_sub(area->nr_pages, &nr_vmalloc_pages);
+       } else if (deallocate_pages == 2) {
+               release_pages(area->pages, area->nr_pages);
+       }
 
+       if (deallocate_pages) {
+               atomic_long_sub(area->nr_pages, &nr_vmalloc_pages);
                kvfree(area->pages);
        }
@@ -2369,6 +2373,14 @@ void vunmap(const void *addr)
 }
 EXPORT_SYMBOL(vunmap);
 
+void vunmap_put_pages(const void *addr)
+{
+       BUG_ON(in_interrupt());
+       might_sleep();
+       if (addr)
+               __vunmap(addr, 2);
+}
+
 /**
  * vmap - map an array of pages into virtually contiguous space
  * @pages: array of page pointers

only with kernel-doc and so on.  I bet somebody has a better idea for a name.