Re: [PATCH] arm: dma-mapping: fix potential endless loop in __dma_page_dev_to_cpu()

[Matthew Wilcox]

On Mon, Aug 07, 2023 at 05:26:57PM +0200, Marek Szyprowski wrote:
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index 70cb7e63a9a5..02250106e5ed 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -718,7 +718,7 @@ static void __dma_page_dev_to_cpu(struct page *page, unsigned long off,
>  			folio = folio_next(folio);
>  		}
>  
> -		while (left >= (ssize_t)folio_size(folio)) {
> +		while (left && left >= (ssize_t)folio_size(folio)) {
>  			set_bit(PG_dcache_clean, &folio->flags);
>  			left -= folio_size(folio);
>  			folio = folio_next(folio);

I've been thinking about this and I think this is the right fix for the
wrong reason.  I don't understand how it can produce the failure you
saw, but we shouldn't be calling folio_next() if left is zero, let alone
calling folio_size() on it.  So I'd rather see this fix:

		while (left >= (ssize_t)folio_size(folio)) {
			set_bit(PG_dcache_clean, &folio->flags);
			left -= folio_size(folio);
+			if (!left)
+				break;
			folio = folio_next(folio);
		}