[PATCH 08/17] prmem: struct page: track vmap_area

[Matthew Wilcox]

On Wed, Oct 24, 2018 at 12:34:55AM +0300, Igor Stoppa wrote:
> The connection between each page and its vmap_area avoids more expensive
> searches through the btree of vmap_areas.

Typo -- it's an rbtree.

> +++ b/include/linux/mm_types.h
> @@ -87,13 +87,24 @@ struct page {
>  			/* See page-flags.h for PAGE_MAPPING_FLAGS */
>  			struct address_space *mapping;
>  			pgoff_t index;		/* Our offset within mapping. */
> -			/**
> -			 * @private: Mapping-private opaque data.
> -			 * Usually used for buffer_heads if PagePrivate.
> -			 * Used for swp_entry_t if PageSwapCache.
> -			 * Indicates order in the buddy system if PageBuddy.
> -			 */
> -			unsigned long private;
> +			union {
> +				/**
> +				 * @private: Mapping-private opaque data.
> +				 * Usually used for buffer_heads if
> +				 * PagePrivate.
> +				 * Used for swp_entry_t if PageSwapCache.
> +				 * Indicates order in the buddy system if
> +				 * PageBuddy.
> +				 */
> +				unsigned long private;
> +				/**
> +				 * @area: reference to the containing area
> +				 * For pages that are mapped into a virtually
> +				 * contiguous area, avoids performing a more
> +				 * expensive lookup.
> +				 */
> +				struct vmap_area *area;
> +			};

Not like this.  Make it part of a different struct in the existing union,
not a part of the pagecache struct.  And there's no need to use ->private
explicitly.

> @@ -1747,6 +1750,10 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
>  	if (!addr)
>  		return NULL;
>  
> +	va = __find_vmap_area((unsigned long)addr);
> +	for (i = 0; i < va->vm->nr_pages; i++)
> +		va->vm->pages[i]->area = va;

I don't like it that you're calling this for _every_ vmalloc() caller
when most of them will never use this.  Perhaps have page->va be initially
NULL and then cache the lookup in it when it's accessed for the first time.