linux-kernel - RE: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for allocations >= PAGE

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Date: Thu, 29 Feb 2024 06:07:32 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Will Deacon <will@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "kernel-team@...roid.com" <kernel-team@...roid.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, Christoph Hellwig
	<hch@....de>, Marek Szyprowski <m.szyprowski@...sung.com>, Robin Murphy
	<robin.murphy@....com>, Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Dexuan Cui <decui@...rosoft.com>, Nicolin Chen <nicolinc@...dia.com>
Subject: RE: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for
 allocations >= PAGE_SIZE

From: Will Deacon <will@...nel.org> Sent: Wednesday, February 28, 2024 5:40 AM
> 
> For swiotlb allocations >= PAGE_SIZE, the slab search historically
> adjusted the stride to avoid checking unaligned slots. However, this is
> no longer needed now that the code around it has evolved and the
> stride is calculated from the required alignment.
> 
> Either 'alloc_align_mask' is used to specify the allocation alignment or
> the DMA 'min_align_mask' is used to align the allocation with 'orig_addr'.
> At least one of these masks is always non-zero.

I think the patch is correct, but this justification is not.  alloc_align_mask
and the DMA min_align_mask are often both zero.  While the NVMe
PCI driver sets min_align_mask, SCSI disk drivers do not (except for the
Hyper-V synthetic SCSI driver).   When both are zero, presumably
there are no alignment requirements, so a stride of 1 is appropriate.

Michael

> 
> In light of that, remove the redundant (and slightly confusing) check.
> 
> Link: https://lore.kernel.org/all/SN6PR02MB4157089980E6FC58D5557BCED4572@SN6PR02MB4157.namprd02.prod.outlook.com/
> Reported-by: Michael Kelley <mhklinux@...look.com>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
>  kernel/dma/swiotlb.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c381a7ed718f..0d8805569f5e 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1006,13 +1006,6 @@ 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;
> --
> 2.44.0.rc1.240.g4c46232300-goog