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Message-ID:
<SN6PR02MB4157DBE41292B3E1B380D9F0D4272@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 8 Mar 2024 19:02:21 +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 v6 6/6] swiotlb: Reinstate page-alignment for mappings >=
PAGE_SIZE
From: Will Deacon <will@...nel.org> Sent: Friday, March 8, 2024 7:28 AM
>
> 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.
>
> Reported-by: Michael Kelley <mhklinux@...look.com>
> Signed-off-by: Will Deacon <will@...nel.org>
Reviewed-by: Michael Kelley <mhklinux@...look.com>
> ---
> 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;
> --
> 2.44.0.278.ge034bb2e1d-goog
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