Re: [PATCH v6 6/6] swiotlb: Reinstate page-alignment for mappings >= PAGE_SIZE

[Robin Murphy <robin.murphy@arm.com>]

On 2024-03-08 3:28 pm, Will Deacon wrote:
> For swiotlb allocations >= PAGE_SIZE, the slab search historically
> adjusted the stride to avoid checking unaligned slots. This had the
> side-effect of aligning large mapping requests to PAGE_SIZE, but that
> was broken by 0eee5ae10256 ("swiotlb: fix slot alignment checks").
> 
> Since this alignment could be relied upon drivers, reinstate PAGE_SIZE
> alignment for swiotlb mappings >= PAGE_SIZE.

This seems clear enough to keep me happy now, thanks! And apologies that 
I managed to confuse even myself in the previous thread...

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Reported-by: Michael Kelley <mhklinux@outlook.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>   kernel/dma/swiotlb.c | 18 +++++++++++-------
>   1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c381a7ed718f..c5851034523f 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -992,6 +992,17 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>   	BUG_ON(!nslots);
>   	BUG_ON(area_index >= pool->nareas);
>   
> +	/*
> +	 * Historically, swiotlb allocations >= PAGE_SIZE were guaranteed to be
> +	 * page-aligned in the absence of any other alignment requirements.
> +	 * 'alloc_align_mask' was later introduced to specify the alignment
> +	 * explicitly, however this is passed as zero for streaming mappings
> +	 * and so we preserve the old behaviour there in case any drivers are
> +	 * relying on it.
> +	 */
> +	if (!alloc_align_mask && !iotlb_align_mask && alloc_size >= PAGE_SIZE)
> +		alloc_align_mask = PAGE_SIZE - 1;
> +
>   	/*
>   	 * Ensure that the allocation is at least slot-aligned and update
>   	 * 'iotlb_align_mask' to ignore bits that will be preserved when
> @@ -1006,13 +1017,6 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>   	 */
>   	stride = get_max_slots(max(alloc_align_mask, iotlb_align_mask));
>   
> -	/*
> -	 * For allocations of PAGE_SIZE or larger only look for page aligned
> -	 * allocations.
> -	 */
> -	if (alloc_size >= PAGE_SIZE)
> -		stride = umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
> -
>   	spin_lock_irqsave(&area->lock, flags);
>   	if (unlikely(nslots > pool->area_nslabs - area->used))
>   		goto not_found;