Re: [RFC] vmap_folio()

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Aug 18, 2022 at 10:10:14PM +0100, Matthew Wilcox wrote:
> 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
> +
Looks pretty straightforward. One thing though, if we can combine it
together with vmap(), since it is a copy paste in some sense, say to
have something __vmap() to reuse it in the vmap_folio() and vmap().

But that is just a thought.

--
Uladzislau Rezki