RE: [PATCH v6 1/6] swiotlb: Fix double-allocation of slots due to broken alignment handling

[Michael Kelley]

From: Will Deacon <will@xxxxxxxxxx> Sent: Friday, March 8, 2024 7:28 AM
> 
> 
> Fix the problem by treating the allocation alignment separately to any
> additional alignment requirements from the device, using the maximum
> of the two as the stride to search the buffer slots and taking care
> to ensure a minimum of page-alignment for buffers larger than a page.
> 
> This also resolves swiotlb allocation failures occuring due to the
> inclusion of ~PAGE_MASK in 'iotlb_align_mask' for large allocations and
> resulting in alignment requirements exceeding swiotlb_max_mapping_size().
> 
> Fixes: bbb73a103fbb ("swiotlb: fix a braino in the alignment check fix")
> Fixes: 0eee5ae10256 ("swiotlb: fix slot alignment checks")
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Reviewed-by: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> Tested-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
>  kernel/dma/swiotlb.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index b079a9a8e087..2ec2cc81f1a2 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -982,7 +982,7 @@ static int swiotlb_search_pool_area(struct device
> *dev, struct io_tlb_pool *pool
>  		phys_to_dma_unencrypted(dev, pool->start) & boundary_mask;
>  	unsigned long max_slots = get_max_slots(boundary_mask);
>  	unsigned int iotlb_align_mask =
> -		dma_get_min_align_mask(dev) | alloc_align_mask;
> +		dma_get_min_align_mask(dev) & ~(IO_TLB_SIZE - 1);
>  	unsigned int nslots = nr_slots(alloc_size), stride;
>  	unsigned int offset = swiotlb_align_offset(dev, orig_addr);
>  	unsigned int index, slots_checked, count = 0, i;
> @@ -993,19 +993,18 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>  	BUG_ON(!nslots);
>  	BUG_ON(area_index >= pool->nareas);
> 
> +	/*
> +	 * For mappings with an alignment requirement don't bother looping to
> +	 * unaligned slots once we found an aligned one.
> +	 */
> +	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)
> -		iotlb_align_mask |= ~PAGE_MASK;
> -	iotlb_align_mask &= ~(IO_TLB_SIZE - 1);
> -
> -	/*
> -	 * For mappings with an alignment requirement don't bother looping to
> -	 * unaligned slots once we found an aligned one.
> -	 */
> -	stride = (iotlb_align_mask >> IO_TLB_SHIFT) + 1;
> +		stride = umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
> 
>  	spin_lock_irqsave(&area->lock, flags);
>  	if (unlikely(nslots > pool->area_nslabs - area->used))
> @@ -1015,11 +1014,14 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>  	index = area->index;
> 
>  	for (slots_checked = 0; slots_checked < pool->area_nslabs; ) {
> -		slot_index = slot_base + index;
> +		phys_addr_t tlb_addr;
> 
> -		if (orig_addr &&
> -		    (slot_addr(tbl_dma_addr, slot_index) &
> -		     iotlb_align_mask) != (orig_addr & iotlb_align_mask)) {
> +		slot_index = slot_base + index;
> +		tlb_addr = slot_addr(tbl_dma_addr, slot_index);
> +
> +		if ((tlb_addr & alloc_align_mask) ||
> +		    (orig_addr && (tlb_addr & iotlb_align_mask) !=
> +				  (orig_addr & iotlb_align_mask))) {
>  			index = wrap_area_index(pool, index + 1);
>  			slots_checked++;
>  			continue;
> --

Question for IOMMU folks:  alloc_align_mask is set only in
iommu_dma_map_page(), using the IOMMU granule size.
Can the granule ever be larger than PAGE_SIZE?  If so,
swiotlb_search_pool_area() can fail to find slots even when
the swiotlb is empty.

The failure happens when alloc_align_mask is larger than
PAGE_SIZE and the alloc_size is the swiotlb max of 256 Kbytes
(or even a bit smaller in some cases). The swiotlb memory
pool is allocated in swiotlb_memblock_alloc() with PAGE_SIZE
alignment.  On x86/x64, if alloc_align_mask is 8191 and the
pool start address is something like XXXX1000, slot 0 won't
satisfy alloc_align_mask.  Slot 1 satisfies alloc_align_mask,
but has a size of 127 slots and can't fulfill a 256 Kbyte request.
The problem repeats through the entire swiotlb and the
allocation fails.

Updating swiotlb_memblock_alloc() to use an alignment of
IO_TLB_SIZE * IO_TLB_SEGSIZE (i.e., 256 Kbytes) solves the
problem for all viable configurations.

Michael