Re: [PATCH 06/10] dma: Use free_decrypted_pages()

[Edgecombe, Rick P]

Link to whole series:
https://lore.kernel.org/lkml/20231017202505.340906-1-rick.p.edgecombe@xxxxxxxxx/

On Wed, 2023-10-18 at 08:24 +0200, Christoph Hellwig wrote:
> On Tue, Oct 17, 2023 at 01:25:01PM -0700, Rick Edgecombe wrote:
> >   struct cma;
> >   
> > @@ -165,7 +166,7 @@ static inline struct page
> > *dma_alloc_contiguous(struct device *dev, size_t size,
> >   static inline void dma_free_contiguous(struct device *dev, struct
> > page *page,
> >                 size_t size)
> >   {
> > -       __free_pages(page, get_order(size));
> > +       free_decrypted_pages((unsigned long)page_address(page),
> > get_order(size));
> 
> CMA can be highmem, so this won't work totally independent of what
> free_decrypted_pages actually does.  Also please avoid the overly
> long line.

Argh, yes this is broken for highmem. Thanks for pointing it out.

For x86, we don't need to worry about doing set_memory_XXcrypted() with
highmem. Checking the Kconfig logic around the other
set_memory_XXcrypted() implementations:
s390 - Doesn't support HIGHMEM
powerpc - Doesn't support set_memory_XXcrypted() and HIGHMEM together

So that would mean set_memory_encrypted() is not needed on the HIGHMEM
configs (i.e. it's ok if there is no virtual mapping at free-time,
because it can skip the conversion work).

So free_decrypted_pages() could be changed to not disturb the HIGHMEM
configs, like this:
static inline void free_decrypted_pages(struct page *page, int order)
{
	void *addr = page_address(page);
	int ret = 0;

	if (addr)
		ret = set_memory_encrypted(addr, 1 << order);

	if (ret) {
		WARN_ONCE(1, "Failed...\n");
		return;
	}
	__free_pages(page, get_order(size));
}

Or we could just fix all the callers to open code the right logic. The
free_decrypted_pages() helper is not really saving code across the
series, and only serving to help callers avoid re-introducing the
issue. But I'm sort of worried it will be easy to do just that. Hmm...