Re: [PATCH 03/12] swiotlb-xen: maintain slab count properly

[Stefano Stabellini <sstabellini@kernel.org>]

On Tue, 7 Sep 2021, Jan Beulich wrote:
> Generic swiotlb code makes sure to keep the slab count a multiple of the
> number of slabs per segment. Yet even without checking whether any such
> assumption is made elsewhere, it is easy to see that xen_swiotlb_fixup()
> might alter unrelated memory when calling xen_create_contiguous_region()
> for the last segment, when that's not a full one - the function acts on
> full order-N regions, not individual pages.
> 
> Align the slab count suitably when halving it for a retry. Add a build
> time check and a runtime one. Replace the no longer useful local
> variable "slabs" by an "order" one calculated just once, outside of the
> loop. Re-use "order" for calculating "dma_bits", and change the type of
> the latter as well as the one of "i" while touching this anyway.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -106,27 +106,26 @@ static int is_xen_swiotlb_buffer(struct
>  
>  static int xen_swiotlb_fixup(void *buf, unsigned long nslabs)
>  {
> -	int i, rc;
> -	int dma_bits;
> +	int rc;
> +	unsigned int order = get_order(IO_TLB_SEGSIZE << IO_TLB_SHIFT);
> +	unsigned int i, dma_bits = order + PAGE_SHIFT;
>  	dma_addr_t dma_handle;
>  	phys_addr_t p = virt_to_phys(buf);
>  
> -	dma_bits = get_order(IO_TLB_SEGSIZE << IO_TLB_SHIFT) + PAGE_SHIFT;
> +	BUILD_BUG_ON(IO_TLB_SEGSIZE & (IO_TLB_SEGSIZE - 1));
> +	BUG_ON(nslabs % IO_TLB_SEGSIZE);
>  
>  	i = 0;
>  	do {
> -		int slabs = min(nslabs - i, (unsigned long)IO_TLB_SEGSIZE);
> -
>  		do {
>  			rc = xen_create_contiguous_region(
> -				p + (i << IO_TLB_SHIFT),
> -				get_order(slabs << IO_TLB_SHIFT),
> +				p + (i << IO_TLB_SHIFT), order,
>  				dma_bits, &dma_handle);
>  		} while (rc && dma_bits++ < MAX_DMA_BITS);
>  		if (rc)
>  			return rc;
>  
> -		i += slabs;
> +		i += IO_TLB_SEGSIZE;
>  	} while (i < nslabs);
>  	return 0;
>  }
> @@ -210,7 +209,7 @@ retry:
>  error:
>  	if (repeat--) {
>  		/* Min is 2MB */
> -		nslabs = max(1024UL, (nslabs >> 1));
> +		nslabs = max(1024UL, ALIGN(nslabs >> 1, IO_TLB_SEGSIZE));
>  		bytes = nslabs << IO_TLB_SHIFT;
>  		pr_info("Lowering to %luMB\n", bytes >> 20);
>  		goto retry;
> @@ -245,7 +244,7 @@ retry:
>  		memblock_free(__pa(start), PAGE_ALIGN(bytes));
>  		if (repeat--) {
>  			/* Min is 2MB */
> -			nslabs = max(1024UL, (nslabs >> 1));
> +			nslabs = max(1024UL, ALIGN(nslabs >> 1, IO_TLB_SEGSIZE));
>  			bytes = nslabs << IO_TLB_SHIFT;
>  			pr_info("Lowering to %luMB\n", bytes >> 20);
>  			goto retry;
>