RE: [PATCH] dma: swiotlb: Round restricted DMA pool size down to IO_TLB_SEGSIZE

[Michael Kelley <mhklinux@outlook.com>]

From: Marek Szyprowski <m.szyprowski@samsung.com> Sent: Tuesday, November 3, 2026 7:50 AM
> 
> All other paths creating a swiotlb pool ensure that nslabs is a multiple
> of IO_TLB_SEGSIZE, and the swiotlb code relies on that. Since commit
> 0b84e4f8b793 ("swiotlb: Add restricted DMA pool initialization") a
> restricted pool can have an arbitrary size taken from the device tree.
> 
> If the pool size is not a multiple of IO_TLB_SEGSIZE, the last segment is
> partial and swiotlb_release_slots() reads mem->slots[index + nslabs],
> which is one past the end of the slots array when the freed buffer ends
> at the end of the pool:
> 
>  BUG: KASAN: slab-out-of-bounds in swiotlb_release_slots
>  Read of size 2
> 
> Round the pool size down to a multiple of IO_TLB_SEGSIZE, wasting a bit
> of memory, and warn about it.

I'm not familiar with the macro scenarios where a device has a
restricted pool specified in DT. But I wonder if it would be better to
round up instead of down. DT has specified that the device should
have a restricted DMA pool of a certain size, and rounding down gives
the device a smaller pool than was requested. Could DMA operations
then fail because of not having a large enough pool? Rounding down
also allows the pool to end up being size zero, which this patch
detects and results in a definite failure at device initialization time.

What's the tradeoff in rounding up instead? The device restricted
pool at least as large as requested. Presumably, being larger than
requested doesn't cause any problems for the device or its driver.
The downside is using up to 256 KiB more system memory than
expected. In an embedded system with only a few MiB of system
memory, maybe that's a relatively big problem. I don't know the
scenarios well enough to judge whether using a bit more system
memory is better or worse than the device not having all the
restricted pool space it said it needed for DMA.

Your commit message also mentions "wasting a bit of memory".
What is wasted in the round down case? The rounding down is
done before the pool memory is allocated. I would have thought
the wasting would occur in the round up case, where the restricted
pool is given more memory than the device said it needed.

Michael

> 
> Reported-by: Giorgi Kobakhia <gkobakhi@asu.edu>
> Closes: https://lore.kernel.org/all/20261026032007.1957949-1-gkobakhi@asu.edu/
> Fixes: 0b84e4f8b793 ("swiotlb: Add restricted DMA pool initialization")
> CC: stable@vger.kernel.org
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
>  kernel/dma/swiotlb.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 2c57b49ac7c0..a68f35192582 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -2007,6 +2007,19 @@ static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
>  	 */
>  	if (!mem) {
>  		struct io_tlb_pool *pool;
> +		unsigned long aligned_nslabs = ALIGN_DOWN(nslabs, IO_TLB_SEGSIZE);
> +
> +		if (aligned_nslabs != nslabs) {
> +			dev_warn(dev, "Restricted DMA pool size %zu is not a multiple of %lu, rounding down to %lu bytes\n",
> +				 (size_t)rmem->size,
> +				 (unsigned long)IO_TLB_SEGSIZE << IO_TLB_SHIFT,
> +				 aligned_nslabs << IO_TLB_SHIFT);
> +			nslabs = aligned_nslabs;
> +		}
> +		if (!nslabs) {
> +			dev_err(dev, "Restricted DMA pool is too small\n");
> +			return -EINVAL;
> +		}
> 
>  		mem = kzalloc_obj(*mem);
>  		if (!mem)
> @@ -2034,7 +2047,7 @@ static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
> 
>  			mem->cc_shared = true;
>  			ret = set_memory_decrypted((unsigned long)phys_to_virt(rmem->base),
> -						   rmem->size >> PAGE_SHIFT);
> +						   (nslabs << IO_TLB_SHIFT) >> PAGE_SHIFT);
>  			if (ret) {
>  				dev_err(dev, "Failed to decrypt restricted DMA pool\n");
>  				kfree(pool->areas);
> --
> 2.43.0