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

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

On 2024-03-08 4:38 pm, Petr Tesařík wrote:
> On Fri, 8 Mar 2024 16:08:01 +0000
> Robin Murphy <robin.murphy@arm.com> wrote:
> 
>> 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>
> 
> I thought we agreed that this stricter alignment is unnecessary:
> 
>    https://lore.kernel.org/linux-iommu/20240305140833.GC3659@lst.de/

No, that was about dma_alloc_coherent() again (and TBH I'm not sure we 
should actually relax it anyway, since there definitely are callers who 
rely on size-alignment beyond PAGE_SIZE, however they're typically going 
to be using the common implementations which end up in alloc_pages() or 
CMA and so do offer that, rather than the oddball ones which don't - 
e.g. we're never going to be allocating SMMUv3 Stream Tables out of some 
restricted pool via the emergency swiotlb_alloc() path). If anywhere, 
the place to argue that point would be patch #3 (which as mentioned I'd 
managed to forget about before...)

This one's just about preserving a SWIOTLB-specific behaviour which has 
the practical effect of making SWIOTLB a bit less visible to dma_map_*() 
callers. The impact of keeping this is fairly low, so seems preferable 
to the risk of facing issues 2 or 3 years down the line when someone 
finally upgrades their distro and their data gets eaten because it turns 
out some obscure driver should really have been updated to use 
min_align_mask.

Thanks,
Robin.

> But if everybody else wants to have it...
> 
> Petr T
> 
>>> 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;
>>
>