Re: [PATCH 6/7] xen-gntdev: Support mapping in HVM domains

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

> @@ -284,8 +304,25 @@ static void unmap_grant_pages(struct grant_map *map, int offset, int pages)
>  		goto out;
>  
>  	for (i = 0; i < pages; i++) {
> +		uint32_t check, *tmp;
>  		WARN_ON(unmap_ops[i].status);
> -		__free_page(map->pages[offset+i]);
> +		if (!map->pages[i])
> +			continue;
> +		/* XXX When unmapping, Xen will sometimes end up mapping the GFN
> +		 * to an invalid MFN. In this case, writes will be discarded and
> +		 * reads will return all 0xFF bytes. Leak these unusable GFNs
> +		 * until a way to restore them is found.
> +		 */
> +		tmp = kmap(map->pages[i]);
> +		tmp[0] = 0xdeaddead;
> +		mb();
> +		check = tmp[0];
> +		kunmap(map->pages[i]);
> +		if (check == 0xdeaddead)
> +			__free_page(map->pages[i]);
> +		else if (debug)
> +			printk("%s: Discard page %d=%ld\n", __func__,
> +				i, page_to_pfn(map->pages[i]));

Whoa. Any leads to when the "sometimes" happens? Does the status report an
error or is it silent?

>  		map->pages[offset+i] = NULL;
>  		map->pginfo[offset+i].handle = 0;
>  	}