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

[Petr Tesařík]

On Fri, 8 Mar 2024 17:17:51 +0000
Robin Murphy <robin.murphy@xxxxxxx> wrote:

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

Sure, swiotlb_alloc() ensures that alloc_align_mask is non-zero, so the
condition in this patch cannot be met. In fact, that's why it could be
moved up to swiotlb_tbl_map_single().

> 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.

The impact is indeed negligible with 4K pages. It may put a bit of
stress on the SWIOTLB with 64K pages, but if the mapping size somehow
correlates with page size, such drivers would need a larger SWIOTLB
anyway.

I had some doubts if there are any guarantees at all for dma_map_*().
Now I see that the email I linked dealt with dma_alloc_coherent().

OK, let's not open the other discussion now.

Petr T

> Thanks,
> Robin.
> 
> > But if everybody else wants to have it...
> > 
> > Petr T
> >   
> >>> Reported-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> >>> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> >>> ---
> >>>    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;  
> >>  
> >   
>